A Treasure Map Could Be Your Next Adventure Hook

Let's do some quick dungeon math. According to the AD&D DMG, 4 in 20 (or 1 in 5) dungeon rooms contain treasure (three guarded by monsters and one unguarded):

When determining exactly what treasure is in a room, you roll on this table once for unguarded treasure and twice for treasure guarded by monsters, adding 10% to the roll in the latter case:

There is a 3% chance of an unguarded treasure room containing magic items and a 26% chance of a treasure room guarded by monsters containing magic items (13% chance because of the +10% to the roll, times two because we're rolling twice). 

Again, 1 in 20 (5%) of rooms will contain unguarded treasure, and 3 in 20 (15%) of rooms will contain treasure guarded by monsters. Thus, 0.15% of all rooms will contain unguarded magic items and 3.9% of all rooms will contain guarded magic items. 4.05% of total rooms will have treasure containing magic items - roughly 4 in 100 or 1 in 25. Interestingly, while all other treasure types scale with dungeon level, number of magic items does not.

Here's what I'm interested in - 10% of magic items are actually maps. What kind of maps? Treasure maps, of course:

"But wait!" you might say, "This excerpt says 'This system can be used for monsters you place on the outdoor map as well as for monsters randomly encountered by a party exploring the wilderness.' It doesn't say anything about dungeons. And TABLE V.G. above even says to 'Roll once on Magic Items Table' - nothing about maps!"

And you would be right. The MAP OR MAGIC DETERMINATION table likely refers to the rightmost column on the treasure table in the Monster Manual:

These treasure types are typically found only in wilderness monster lairs. Some are found as "individual treasure" (i.e. coinage carried in the pockets of goblins and such), but none of those types have a chance to contain maps.

So there you have it - no treasure maps in dungeons. Except:

This seems to indicate that treasures maps can be found in dungeons. Presumably, when the term "monster's lair" is used in the previous excerpt, it is meant not only to apply to wilderness lairs but also dungeon rooms in which monsters are guarding treasure.

Note that "A map should never list its treasure, only show its location." There will be some guidance later as to where and what kind of treasure the map leads to, but no guidance is provided as to how to actually create a treasure map for your game, "for how can it be possible to direct each DM properly considering the infinite possibilities under which the map will be located?" If you want gameable examples, Phlox has some good ones.

Taking a step back, we know that 3% of unguarded treasure rooms and 26% of guarded treasure rooms contain magic items. Of the guarded treasure rooms containing magic items, 98.31% will contain one magic item and 1.69% will contain two magic items. Considering that 10% of magic items will actually be treasure maps, 0.3% of unguarded treasure rooms containing magic items and 10.169% of guarded treasure rooms containing magic items will contain maps (10% of guarded treasure rooms containing one magic item = 9.831% + 20% of guarded treasure rooms containing two magic items = .338%).

Again, 5% of total rooms contain unguarded treasure and 15% of total rooms contain guarded treasure. 0.015% of total rooms contain unguarded treasure maps and 1.52535% of total rooms contain guarded treasure maps - 1.5255% of total dungeon rooms contain treasure maps. That's 1 in every 66 rooms. If your dungeon is big enough (and particularly if you're aspiring for anything of the "megadungeon" classification), there will probably be a treasure map in there somewhere. I hope I did my math right! I am happy to be corrected.

Note that tricks found in dungeons might also release maps, as found in APPENDIX H: TRICKS:

Treasure maps found in dungeons usually point the way to treasure inside that very dungeon, but might point to "another lost dungeon" or somewhere "along the course of a long underground river."

Let's dig into those wilderness lair treasure types (associated monster types are sourced from here):

• Type A and F hoards (e.g. humans, centaurs, vampires, basilisks, medusae, shadows, rakshasa, rhemoraz, salamanders, leprechauns, and chimeras) have a 9% chance of containing a map
• Type C hoards (e.g. minotaurs, gargoyles, lycanthropes, gnomes, and pixies) have a 2% chance
• Type D hoards (e.g. orcs, hobgoblins, gnolls, mummies, dryads, griffins, chimeras, manticores, and purple worms) have a 3% chance
• Type E hoards (e.g. giants, wraiths, gorgons, and wyverns) have a 7.5% chance
• Type G hoards (e.g. dwarves) have a 14% chance
• Type H hoards (e.g. dragons) have a 6% chance
• Type I hoards (e.g. rocs) have a 1.5% chance
• Type W hoards (e.g. buccaneers) have a flat 55% chance of having a map
• Type Z hoards (e.g. demigods and demon lords) have a 15% chance

Buccaneers, of course, have the greatest chance of possessing a treasure map. There are plenty of genre tropes to draw upon in this case. Demigods and demon lords are next in line. Do the maps in their possession point towards their own secreted away hoards? Dwarves are next, and it's easy enough to imagine why if you're familiar with The Hobbit. But what of the others? Whose treasure maps do these monsters possess, and how did they come to acquire them?

Here's what the DMG says about maps found in wilderness lairs:

Treasure maps found in wilderness lairs can themselves lead elsewhere within the lair (an underground "labyrinth of caves") or to some other location 5 to 500 miles away, which may be buried and unguarded (in the classic way that pirates bury treasure), hidden in water, guarded in another monster's lair, in ruins or a crypt (presumably themselves dungeons), or even "secreted in a town".

In rare cases, the player character might also obtain a treasure map by drawing the Key card from a deck of many things:

Note that treasure maps are written in a cypher. Like a spell scroll, a read magic spell must be employed to decipher one:

This means that treasure maps are also subject to all kinds of trickery on the part of the DM, like curses and traps triggered by reading their contents - explosive runes and magic symbols the like. The text states that "a map will appear magical until the proper spell is used" (emphasis mine), suggesting that the map itself is not necessarily magical.

You can also get your highly-intelligent sword to read the map for you:

Treasure maps come in four types: false maps (which presumably lead to nothing at all), those leading to monetary treasure, those leading to magic treasure, and those leading to "combined hoards" of both money and magic items. Note again that a treasure map should never indicate what it leads to. 

MONETARY TREASURE "is weighted towards large quantities of coins which will require a train to remove - or must be left entirely if foreplanning is not observed":

If not carried away immediately, the DM is encouraged to set a percentage chance that the treasure will be stolen by someone or something else. This is more likely if the treasure was guarded by some other monster, not unlike the Five Armies showing up to claim Smaug's treasure after he was slain. This meshes nicely with my post about treasure as a source of challenge in AD&D, in which the comparison to The Hobbit is made explicit.

Less of an inconvenience will be MAGIC TREASURE, which can be easily carried away:

This table is weighted towards potions, scrolls, armor, and weapons, "so as to prevent imbalance in the game" and to "Keep potent magic items rare." The DM is encouraged to "Increase scarcity by destroying or stealing what is found!"

One might wonder whether the "Any [X amount of magic] items" results might in turn yield another treasure map. I don't think this is intended to be the case, as this possibility is made explicit in the COMBINED HOARD table:

With a result of 81 or higher, you will find the best possible monetary or magic treasure plus a map to either a separate magic treasure or a separate monetary treasure, respectively. Since these are the most valuable hoards, the DM is encouraged to hide them far away and to guard them with traps and monsters. There's something very funny about completing a long and arduous adventure following one of these maps only to immediately come into possession of another.

So what's the point of treasure maps? Well, they're built in adventure hooks! When found in dungeons, they point the players towards some destination within the dungeon or to another dungeon entirely. When found in the wilderness, they can point the players towards some other location in your sandbox or somewhere beyond the current area of play entirely. 

Want the players to go deeper into the dungeon? Give them a treasure map. Want them to visit a different dungeon? Give them a treasure map. Want them to trek out into the wilderness, venture underwater, track down a monster's wilderness lair, or get mixed up in the deadly social fabric of urban gameplay? Give them a treasure map.

If you're flying by the seat of your pants, rolling for treasure as the players find it, a treasure map gives you the chance to detail a new location in which to house the hoard - a fun little surprise for you. It's quantum treasure - even you don't know it exists until it comes into play. Unfortunately, you can't do much with something you don't know about.

If on the other hand you're prepping everything ahead of the campaign's start - including rolling for treasure in dungeons and lairs - then rolling up a treasure map somewhere gives you the chance to hide the hoard from the very beginning. You know there's a massive hoard of treasure buried in an inconspicuous location right beneath the starting town, but the players don't, and they won't know unless they find that map. 

Maybe the treasure hoard is the stuff of legend, and your setting's factions and NPCs are all after it, but no one knows where the map is. Rumor has it that it was lost deep within the dungeon, or that the ancient red dragon counts it among its hoard. The legendary Dread Pirate is rumored to have hidden his plundered goods on one of those steamy jungle isles off the southern coast, but the map was lost when his flagship sunk a decade ago.

Another possibility is that whoever or whatever possesses the map is actively trying to find the treasure. Maybe the evil wizard who rules the town has the map already, but is seeking the means to acquire the hoard their self. A clan of dwarves seeks a burglar to help them reclaim their lost gold. The buccaneers are scouring the coast for something, plundering as they go. What are they looking for? Perhaps the orcs, hobgoblins, or gnolls are harassing human civilization because they know there's treasure buried beneath one of those towns. Perhaps a dragon with a treasure map is responsible for martialing that host to acquire the treasure on its behalf.

Once the player characters have the map, the adventure is only just beginning. They have to plan an expedition into the dungeon, the wilderness, or out to sea. They might have to recruit a caravan of drovers and porters to help them haul the hoard away when they find it. They have to contend with all the usual dangers of the environment its found in, any monsters that guard it, and any traps laid upon it. 

They have to do all of this while somehow keeping it a secret from those same factions and NPCs who are after it, even as the logistics may require recruiting dozens of hirelings or hauling the treasure out in the middle of town right under those same NPCs' noses. Hopefully it isn't a false map. That would be most unfortunate.

Please excuse the pun, but that is a veritable treasure trove of juicy gameplay implications. Go on. What are you waiting for? Put a treasure map in your game right now. 

The Player-Drawn Dungeon Map is a Fast Travel Hack

This post is part of the Prismatic Wasteland blog bandwagon on the topic of maps.

The first mention of mapping by players in the AD&D PHB is on page 101, in the section titled THE ADVENTURE, specifically Dungeon Expeditions:

Here it is explicitly stated that the purpose of the player-drawn dungeon map is to help the party "find [their] way out [of the dungeon] and return for yet more adventuring." There is a tradeoff for mapping the dungeon, which is that movement through the dungeon will be slow. Presumably, this will result in more torches burned, more rations eaten, and more wandering monsters encountered during mapping expeditions.

Just after this, on page 102, we are told that movement through the dungeon is faster when following a map:

Dungeon movement is five times faster when following a previously mapped route. Mapping the dungeon not only allows the players to discern the route back to the surface, but also allows them to move expeditiously along that route (thus reducing the chance of encountering wandering monsters and the use of torches and rations as they retreat).

Page 102 also tells us that mapping is not possible while moving quickly, "such as when pursued or pursuing." Additionally, "light must be available to make or read a map" (infravision doesn't cut it). Marking the walls of the dungeon (as with chalk) or leaving a trail of dropped items or string is "typically useless...as they will be obliterated, moved, or destroyed by passing creatures." The monsters are onto your dirty tricks, and they're here to ensure you end up trapped in the dungeon with them.

The section on MAPPING (page 106) recommends that two players make maps to improve the success of the expedition. It also warns players to "Never become concerned if your map is not exact...As long as it gives your party an idea as to where they are and how to get back, it is serving its purpose."

This coincides neatly with A Knight at the Opera's post on Navigation Games, in which the author argues that an abstracted flowchart-style map which foregoes precise spatial dimensions in favor of relativistic relationships between rooms is the ideal form of mapping in dungeon exploration games.

The most comprehensive remarks on mapping are found in this paragraph under SUCCESSFUL ADVENTURES (page 109):

Here is the crux of the issue: a map ensures that the party will be able to return to the surface. Again it is stated that "Minor mistakes are not very important...so long as the chart allows the group to find its way out!" I will note that the use of the word "chart" calls to mind not a meticulously drafted schematic on graph paper, but something like the flowchart-style map advocated for in A Knight at the Opera's post.

Again it is recommended that two maps be kept in case one is "destroyed by mishap or monster." This is offered alongside sound advice like "In mazes always follow one wall or the other" and things I would never even consider like "[when pursued], always go in a set escape pattern if possible" so your steps can be easily retraced, and "If transported or otherwise lost, begin mapping on a fresh sheet of paper."

That last point brings to mind a room in Dungeon Module B1: In Search of the Unknown in which the party steps into a room and is teleported to another identical room without any indication that they've been transported. I can only imagine the frustration of the hyper-precise graph paper cartographer encountering this room and making a mess of their meticulously measured and sketched map. With a less precise flowchart-style map, the mapper could instead simply add a bubble labeled "teleporting rooms" and connect it to the rest of their chart as they would any other.

In the next paragraph, the value of the map in avoiding encounters with wandering monsters is made explicit:

The map serves another purpose, which is detailed in the following paragraph:

The party is assumed to have an objective, and should not stray from it. "A good referee" will try to distract them from this objective. While it is the party's job to ignore these diversions as much as possible, it is the mapper's job to record their locations so that future expeditions can investigate them. 

The mapper is in many ways the party's memory - they record the dungeon's layout not just so that the party knows where they are and how to quickly escape, but so they know what to come back to. The information they record is instrumental in setting future expedition goals. And, since the party can travel faster through the dungeon along mapped routes, they will be able to quickly make their way back to those diversions on future delves.

Of course, not every one of the DM's nefarious tools can be thwarted by simply ignoring them. The section on TRAPS, TRICKS, AND ENCOUNTERS (page 103) describes traps which confine, channel, injure, or kill characters. Most important to this subject are channeling traps, which include shifting walls and one-way doors. Mapping is suggested as a solution to these traps, as a good map allows the party to find their way back after ending up on the wrong end of one:

The following section on tricks also describes dungeon features which specifically "foul maps", similar to the teleporting rooms in B1:

Let's jump over to the DMG, where the first mention of player mapping is in the section on EXPERT HIRELINGS (page 33):

Specialist scribes (or cartographers) can be hired on a monthly basis to make or copy maps. They demand ten times the fee of a normal scribe, which is 15 gold pieces per month (150 gold pieces per month for a cartographer). Gygax tends to recommend that NPCs charge more to travel somewhere or to provide their services in a dangerous environment, so I imagine that this would be even more expensive if the cartographer is meant to join the party in the dungeon (perhaps five to ten times the usual cost, so 750 to 1,500 gold pieces per month). 

Still, this is well worth the cost if the party can afford it, as they are essentially relieved of the burden of mapping themselves. Instead, I imagine the DM would provide them with a map as they go which, since a well-paid specialist is making it by way of the DM, is probably quite accurate. Of course, a particularly cruel DM is at liberty to take advantage of this by making the cartographer unreliable in some way or by using monsters to target the party's expensive civilian escort.

Note that charmed creatures will not draw maps, per page 43:

If the party wants a dungeon map, they will either have to make it themselves or pay an expensive hireling to do it (and protect said hireling in the process). If they cannot or choose not to circumvent the challenge by paying an NPC, they will themselves have to deal with threats such as traps which channel them into unintended areas and tricks which meddle in their attempts to map. 

We have also been dancing around the issue of time, which imposes the threat of wandering monsters and resource expenditure. So how long does it take to map? The section on THE FIRST DUNGEON ADVENTURE (page 96) gives us some details. This first paragraph suggests that mapping a room or chamber takes 10 minutes:

The next paragraph suggests that each turn, the party can map a length of corridor equal to the party's base movement rate times ten:

Simple enough. However, the first paragraph is contradicted by this next one, which suggests that it takes one turn to map only a 20' x 20' area:

Or perhaps the correct way to read this is that mapping any room or chamber takes one turn, which is the same amount of time it takes to casually examine a 20' x 20' area. I'm not entirely sure.

I tend to let players map one room or chamber per turn. Obviously, this requires stopping in the room or chamber for one turn, so the mapper might update the map while the rest of the party searches the room, checks for secret doors and traps, picks a lock on a door, or whatever. I do kind of like the rule for mapping hallways as I'm never quite sure how to handle those, so maybe I'll keep that one.

There's actually a third way for the players to obtain a map of the dungeon, and that's if they find one. The sample dungeon included in this section of the DMG provides the opportunity for the player characters to find an incomplete map of the dungeon in just the second room:

The importance of this incomplete map is made apparent in the example of play which follows:

The partial map indicates to the players where there might be areas to explore (in this case, they go to Area 3). Since they know that there is an area south of this room, yet there are no exits, they suspect that there must be secret door leading south, and indeed there is.

This, of course, allows the DM to set up an evil trap involving some ghouls:

So maybe if the DM gives you a map of the dungeon in only the second room, be a little wary of where that map might lead!

While there are many benefits and perils when it comes to mapping the dungeon, ultimately the one we must come back to is this: the map helps the party move expeditiously throughout the dungeon. It not only aids the players in remembering the way out of the dungeon, but it hastens them in their egress. The map doesn't just help the players remember things to come back to later, but allows them to return to those side passages more quickly.

Regarding the former - for a player-made dungeon map to yield its true value, getting out of the dungeon without one needs to be every bit as hazardous as making progress into it. This danger is essential to D&D's resource attrition game, which must work as intended in order for the game to remain challenging without relying too heavily on cheap tricks like traps and monsters which can instantly kill the player character. 

One of the most common complaints about D&D's gameplay loop (particularly in post-TSR editions of the game in which all resources are recovered on a single night's sleep) is that the game's attrition-based challenges are too easily circumvented by abusing rests - i.e., the 15-minute adventuring day. This effect is worsened when the DM provides the players with perfect information about the areas of the dungeon they've already explored, such as if the DM draws the map for them as they go, or uses a VTT and leaves previously explored areas of the dungeon displayed on screen after the characters have left them. To rest and recover all their resources, players need only go back the way they came, which is plainly laid out for them by the DM.

If the players have to actually make the map themselves, there's a chance of them doing a poor job. Those who map poorly (or foolishly choose not to map at all) run the risk of getting lost. If they're lost, they can't leave whenever they want to recover lost resources. They may find themselves low on hit points and spells, wandering aimlessly, and easy marks for the dungeons depredations.

The players are rewarded for their efforts not only with hastened escape from the dungeon, but also with expedited travel back into it to tie up those many diversions seeded by the DM on prior expeditions. This allows them to avoid needless encounters with wandering monsters on subsequent delves, making it more likely that returns to the dungeon will be successful and ensuring that the party maintains its momentum throughout the campaign. This doesn't mesh well with modern D&D's dungeons which are seemingly designed to be cleared out in one go, but is essential for making progress in old school D&D's more megalithic underworlds.

The point of the players making the map is to make movement through the dungeon faster, but they have to make it themselves. As argued in A Knight at the Opera's post, mastering the layout of an environment like the dungeon is a perfect example of a challenge of player skill. The map is a powerful tool that must be earned. Don't give it away for free. Or, if you do, be sure to use it to lure the party into an ambush by some ghouls. That'll teach 'em.

Underwater Adventures: D&D's Forgotten Realm

Underwater adventures are grossly underutilized in D&D. Even simple nautical or seafaring adventures (where the main mode of wilderness travel is via boat rather than on foot) are relatively rare. There might be the occasional partly flooded dungeon or passage on a ship to the Isle of Dread or Chult, but little of the adventure in D&D seems to take place beneath the waves themselves.

This makes plenty of sense, as players are likely controlling land-dwelling characters like humans, dwarves, elves, or halflings. "Water levels" in video games like Donkey Kong Country, Legend of Zelda, or Metroid (to name but a few) are iconic, but often for the wrong reasons - that is, they're annoying as Hell. Presumably, underwater adventures in D&D would be likewise. 

The AD&D DMG devotes two full pages to play in these environments (which at once seems like a significant amount while also being unfathomably scant). There's a lot to consider in those two pages. There's also a lot to consider beyond them. But look at this introduction:

Ancient submarine civilizations? Creatures half-man and half-fish? Mountains of sunken loot? Pearls as big as a man's head? Beautiful mermaids? I need to go there. Why don't we go there more?

For starters, there are issues like breathing, movement, vision, combat, and spell use to consider.

 Underwater breathing can be achieved by spells, potions, magic items, or other contrivances invented by the DM for the occasion:

Since the ability to breathe underwater is time-limited, underwater adventures are assumed to be akin in scope to dungeon adventures, rather than wilderness adventures. What I'm imagining is the characters piloting a boat above some sunken ruin, casting all their spells or chugging their potions, and diving in for a short expedition. While all dungeons have time pressure in the form of random encounters and available light sources and provisions, underwater adventures take this to the extreme. If you can't get out in time, you die.

(Add "comestible" to the list of words I learned from reading D&D.)

Just as underwater expeditions are similar to dungeon adventures in terms of duration, they are also similar in terms of movement:

When adventuring underwater, characters use their dungeon movement rate. A character with a movement rate of 12" can move 120 yards per turn outdoors on dry land or 120 feet per turn in a dungeon on dry land. Underwater, they likewise move 120 feet per turn. This assumes that underwater adventures are basically "outdoors".

How quickly then does the character move in an underwater dungeon? It isn't clear. Since your "outdoors" underwater movement rate is equal to your dry land dungeon movement rate (itself one-third of your dry land outdoor movement rate), I would personally rule that your movement rate in an underwater dungeon is likewise one-third your dry land dungeon movement rate. So, a character with a movement rate of 12" would move 40 feet per turn in an underwater dungeon. Sounds pretty reasonable to me.

Swimming is only possible when the character is wearing leather armor or lighter and is carrying fewer than 20 pounds. That's not a lot of equipment! Then again, you probably don't want to get most of that equipment wet anyway. Even magic equipment like a ring of swimming does not permit swimming if these conditions are not met.

Swimming introduces another unique element of underwater adventures - swimming characters are vulnerable to attack from every direction. Underwater adventures are more three-dimensional than land adventures because, outside of flight or levitation, characters don't usually have such unfettered access to that third dimension.

This section also gives us a hint at what sorts of environments or terrain are found underwater - hills, coral outcroppings, shipwrecks, seaweed forests, and the like. Note that swimming without the aid of free action magic does not allow you to move faster than someone forced to walk along the ocean floor, but it does allow you to swim over or around these obstacles.

Next is vision:

There are basically "simple" rules for vision (you can see X' until you reach a depth of Y', at which point the environment is treated basically as darkness) and "complex" ones. (you can see X' when 10' below the surface, but X decreases by 10' for every 10' you descend). 

Underwater environments complicate infravision because the temperature of the water is varied by shifting currents. Seaweed, schools of fish, and mud can obscure vision. Sea grass can be up to 30' tall and clouds of mud can persist for up to 12 rounds!

Then we have combat considerations:

Without free action magic, only the use of thrusting weapons is possible, and natural swimmers will always strike the first blow against landlubbers unless a weapon with reach is employed. 

This is where nets (themselves the unsung heroes of the D&D weapons list) will shine. Characters can throw a net only a number of feet equal to their Strength score, and suffer -4 to hit with them unless they've trained extensively (underwater!) with the use of such a weapon. 

Specially-made crossbows costing ten times the normal amount can also function underwater. If an underwater adventure is to transition to one in an airy environment ("like the great air-filled domes of Atlantis"), bows, missiles, scrolls, and other items must somehow be kept dry.

We also get a taste of what creatures might be encountered in the water - aquatic elves, locathah, mermen, and the true terrors of the deep, sahuagin. The latter employ clever and dastardly net traps in their ambushes.

Finally, spell use:

Like most everything else, spells function as they do in dungeons, with a few caveats. Some material components will not function underwater. Fire-based spells will only function in airy water, and electrical spells will affect the entire area. I imagine it would also be difficult to make the precise gestures required by somatic components (unless magically capable of moving freely) and to speak verbal components (unless in airy water or maybe if magically capable of breathing water), but this isn't stated.

There is a sizeable list of spells that won't function underwater or will only function in airy water (marked by an asterisk):

Many of these are fire spells, but there are other notables - spells which summon insects, and spells dealing with air, wind, or the weather. Animal summoning is curious - could the caster not just summon aquatic animals instead? I imagine this is because the spells and their tables assume that adventures will be happening on land. Still it would be nice if alternative underwater tables were provided. You can't summon a cacodemon or woodland beings, but you can apparently cast monster summoning. Unlike animal summoning, the monster summoning spells do get their own underwater tables:

Also of interest is that spells that require speaking (speak with dead, speak with plants) function only in airy water, which seems to suggest that unless air is present, speaking is not possible. So then wouldn't any spell with verbal components not function without air as well? That's like, most spells. Curiously, speak with animals is omitted. 

I would probably say that if you can breathe underwater, you can speak (and utter verbal components) underwater. If you are capable of swimming (either due to being unencumbered by armor and equipment or by way of magic), you can perform somatic components. Easy peasy.

Then there are a handful of spells which are specially altered when cast underwater:

My favorites have got to be wall of ice (the entire wall simply floats to the surface) and Otiluke's freezing sphere (the caster entombs their self in a sphere of ice, which floats to the surface, and they suffocate unless freed).

This all sounds like a bit of a headache for the DM and players alike, and that's because it is. Underwater environments are alien - more alien even than the dungeon. The deep ocean is probably the most alien place on our own planet Earth. It makes sense why so few adventures take place underwater. Adventuring there should be hard, because the characters aren't from there. But what if they were?

The allure of underwater adventures, in my opinion, is not brief expeditions to sunken dungeons (although that is certainly one of the lures), it's the ancient civilizations and the creatures half-man and half-fish - the aquatic elves, locathah, mermen, sahuagin, and the like. There are vast empires (or probably more accurately in most cases, pretty large isolated ethnic enclaves) beneath the waves, and they are inhabited by all sorts of aquatic people. Can't the players play as one of those people?

Let's start by looking at the underwater encounter tables. These are broken into shallow water (where vision is relatively easy) and deep water (where light from above does not penetrate), and into fresh water and salt water. Who says your underwater adventures have to take place in the ocean?

Interestingly, the paragraph before the tables suggests that the "Number of creatures encountered should be appropriate to the strength of the encountering party":

Hmm...Here I thought old school D&D was supposed to eschew balance (which is maybe true - some true believers don't consider AD&D "old school"). While this same sentiment isn't reiterated prior to the land-based wilderness encounter tables, I can't imagine why it would apply only to underwater encounters. File this under "Important rules mentioned once in passing in a random section of the DMG".

Anyway, I will admit that the fresh water encounter tables do not suggest some larger civilization of which the player characters might be a part or might engage with. It's mostly animals and unintelligent monsters:

Hobgoblins and lizard men are certainly playable races in later editions of D&D. There are some other intelligent monsters in here like giant beavers, gargoyles, ghouls, naga, nixies, and nymphs, but none of those exactly read as playable races (though a giant beaver campaign would be sick). Interestingly, dinosaurs are on the list, despite being found on land only in "Pleistocene Settings". Even if your AD&D world overall is not populated by dinosaurs, they will still dwell in your rivers, lakes, and oceans!

The deep water table is more of the same, though it eschews a few monsters (regular crocodiles, giant frogs of all kinds, green slime, hippopotami, regular lampreys, giant leeches, and nymphs) in favor of others (giant water beetles, dragon turtles, storm giants, giant lampreys, and purple worms). It's cool that the deep lake next to town might have a dragon turtle, a purple worm, or a storm giant's castle at its bottom.

The salt water tables are a bit more interesting:

Here are the aquatic elves, locathah, mermen, and sahuagin. They are joined by ixitxachitl, sea hags, triton, and more nasty sea creatures, plus many of the same interesting creatures from the fresh water tables like storm giants, hobgoblins, gargoyles, ghouls, and nymphs. The deep water table adds the eye of the deep (a deep sea beholder) and one of my favorite weirdos, the morkoth.

Underwater environments in AD&D are full of nasty monsters that leap out of hiding to eat you, swallow you whole, capsize or sink your ship, poison you, paralyze you to be eaten, stun you with light or trick you with illusions to tear you apart with their crab claws, torture you, melt you, suck your blood, enslave you (but only for one year!), and blind you or kill you by looking at you or by getting nude in front of you.

Like many of AD&D's environments, underwater is not a nice place. But these environments are also home to intelligent beings with their own societies. 

10% of dolphins form underwater communities of 4 to 100 dolphins with swordfish and narwhals as trained guards. Storm giants live in spacious castles beneath the waves and keep sea lions as pets (for whatever reason, all of their magical abilities are those select spells which explicitly do not work underwater). Koalinth (aquatic hobgoblins) fulfill much the same function as their land-dwelling counterparts. Ixitxachitl lair in secret coral reef caves and can be clerics of as high as 8th level. Some of them are vampires. Locathah live in hollowed out castle-like undersea rocks, ride eels into battle, and use Portuguese men o' war as traps (implying the existence of Portugal in the default AD&D setting). Water naga are curious creatures capable of casting spells as 5th level magic-users.

Aquatic elves are much like land-dwelling elves (with whom they engage in trade), but they are allies of dolphins and mortal enemies of sharks and sahuagin. There are wars being waged beneath the waves! They dislike fishermen because the elves get caught in the fishermen's nets, are mistaken for sahuagin, and killed.

Mermen live in undersea communities among the reefs and cliffs, sometimes constructing villages of shells, rocks, and coral. They herd fish, have their own workshops, and keep barracuda as pets. Oh, they also form raiding parties to grapple surface ships en masse and slowly drag them down to the bottom of the sea to be looted, killing everyone aboard in the process. Huh.

Sahuagin get more than a full page all to themselves. They are devil-worshippers and dwell "in a vast undersea city" ruled by a king. The city is built in an undersea canyon lined with palaces and dwellings. 5,000 sahuagin dwell there with 1,000 queens, concubines, nobles, and guards in the king's retinue. The king rules nine provinces (mirroring the levels of Hell), each ruled by a prince. The princes live in strongholds, while most other lairs are "actual villages or towns, constructed of stone" and hidden among the seaweed. 

They align themselves with sharks, venture onto land to raid, and hate even the evil ixitxachitl. They have a matriarchal religious structure with clerics of up to 8th level. 1 in 216 (2+1+6=9?) - including the nine princes and the king - is a mutant with four fully functioning arms. Those they capture are either tortured and eaten or made to compete in blood sport, dying either way.

They are either distant relatives of the sea elves created by the drow, or some Lawful Evil god created them from humans "when the deluge came upon the earth." I'm sorry - I wasn't aware of The Deluge! This is rich stuff.

Triton live in undersea castles and sculpted caverns, can become 6th level magic-users, and keep hippocampi, giant sea horses, and sea lions as pets. They can use their conch shell horns to summon these creatures to their aid and panic unintelligent sea creatures. Some have psionic ability. They are from the Elemental Plane of Water and "have been planted on the Material Plane for some purpose presently unknown to man", engaging in wars with sahuagin, ixitxachitl, koalinths, and lacedons.

There's a lot going on underwater! It's dangerous and comes with a great many complications, so I can see why humans, land elves, dwarves, and halflings aren't exactly eager to board a ship and risk getting sunk by a dragon turtle, giant squid, or raiding party of murderous mermen, let alone actually go diving down there to contend with these horrible creatures in such an alien environment. That doesn't mean we can't have adventures there.

I could see a campaign where the player characters are aquatic elves, locathah, mermen, tritons - hell, even koalinth or sahuagin - plundering the ruins of their own ancient civilizations as well as the sunken remains of the civilization above, building undersea strongholds among the coral and cliffs, and getting mixed up in factional politics and aquatic warfare. There are still dungeons there, and dragons, too (in turtle form, at least) - the only two things you really need for a D&D campaign.

A campaign in such a drastically different environment would need to have many considerations. Being able to swim freely underwater is akin to every creature having unlimited flight. I imagine no one is wearing armor. How do potions and scrolls and spellbooks work? Have the magic-users of the undersea realms developed an underwater version of fireball that cooks you alive in a blast of boiling water? That sounds pretty neat.

I need to go there. Why don't we go there more? Why aren't we there right now?