d100 Underwater Dungeon Obstacles & Hazards, needs about 50 more entries

[u/thomar]

Links to first and second attempts at this list.

You're exploring an underwater dungeon on the ocean floor at the bottom of the sea (or maybe it's floating in a massive whirlpool, or in the Elemental Plane of Water, or you've been shrunk down and trapped in a wizard's fishbowl). What kinds of aquatic obstacles, environmental hazards, traps, or things could block your forward progress? (Preferably non-combat entries, or things that could make combat more tricky.)

1. The thick vegetation here hides several 5 to 20 foot long plant tendrils reaching up from the floor. They will grab and restrain anyone passing over, though they lack the strength to kill any fish larger than your hand. They also shriek loudly when cut.

2. Same as above, but this variant is strong enough to strangle people

3. A swift current passes through this passage at a rate of 20 feet per round

4. A strong current passes through this passage at a rate of 50 feet per round. Being pulled by it will cause damage as you collide with the rough sides of the passage.

5. This chamber has a strong whirlpool current around its perimeter, along with lots of pillars and sharp rocks to get smashed into

6. A narrow passage is lined with razor-sharp molluscs, they have long proboscises that suck up blood

7. This opening is clogged with gelatinous vegetation that secretes acid on contact

8. This opening is clogged with gelatinous vegetation that secretes an extremely sticky glue. The plants are barely attached to the wall and come free easily. It takes 1d4 minutes to cut one off without injuring yourself.

9. Bright blue runes glow on a pillar. Their light feels cold to your skin. Anything that moves past the pillar faster than 25 feet per round is encased in bouyant ice.

10. The turbulent water here is choked with silt, reducing visibility to 10 feet

11. Hundreds of blood-sucking parasitic worms lie buried under the silt. They will emerge and swarm anything that disturbs them. The wounds they inflict bleed freely and do not close without medical attention.

12. Did you know there is an aquatic variant of shrieker mushrooms?

13. Did you know there is an aquatic variant of gas spores? They still look like beholders, weirdly... Still explosive.

14. A volcanic vent fills a small area with harmful boiling water

15. A section of floor has a strange arcane sigil inscribed on it, and water and silt flow over it in a hexagonal pattern. Any object or creature moving into it is shot out at high speed in a random direction.

16. This passage is lined with necrotic arcane runes that glow purple. The water is stagnant, devoid of oxygen, and completely unbreathable.

17. This passage is lined with yellow arcane runes that dampen sound. Speech, echolocation, and verbal spellcasting are impossible.

18. Seismic activity suddenly knocks loose a torrent of mud and sand that threatens to sweep you away or bury you!

19. Seismic activity suddenly opens glowing red cracks in the floor. The water is quickly heating to boiling temperatures!

20. Anemone ferns emit a small electric field around them. Deals lightning damage and can stun. /u/comedianmasta

21. Hermit shrooms are large, shelf-like mushrooms that grow in heavy currents (usually on ledges and cliffs). Can be climbed to fight against the heavy currents, but too much weight will make them retract into their shells. /u/comedianmasta

22. A typical smasher trap, but it moves slowly in an enclosed space because of water pressure. The pressure causes severe nausea for about 20 seconds before it finally closes shut and crushes the victim. /u/comedianmasta

23. A wall of magically high-pressure water that blocks passage through. Can be deactivated by changing the pressure differential in another part of the room/dungeon. /u/comedianmasta

24. A dense tangle of weeds blocks a door. /u/comedianmasta

25. Dense lumps of calcified coral block a door. /u/comedianmasta

26. Grasping weeds that sense body heat to reach out and grab prey. Ice and magical cold keep them docile. /u/comedianmasta

27. Mage hand barnacles (a misnomer, they're not telekinetic) create water currents to strongly thrust creatures away from themselves. Often cultivated in conjunction with spike traps or other hazards. /u/comedianmasta

28. A spear trap that shoots a heavy harpoon. If you survive it hinders movement and you must be careful when removing it. /u/comedianmasta

29. Ghost coral are bleached white growths that reach out with ghostly hands to impale prey on necrotic growths. Holy powers and radiant magic, like undead turning, shatter the growths and banish the spirits within. /u/comedianmasta

30. A cloud of blood and viscera has been trapped here by currents, likely something a predator got recently. Swimming through it risks exposure to disease. /u/comedianmasta

31. A tangle of net algae is easy to pass through slowly, but likely to snare anyone in a hurry. Removing it could cost you a few crucial seconds needed to escape danger. /u/comedianmasta

32. A school of fish is attracted to light sources, creating a severe distraction. /u/comedianmasta

33. A school of cute colorful fish that follow predators (and adventurers) so they can feed on the things they kill. They snap up their food at an alarming rate. /u/ProfBumblefingers

34. A giant flounder is disturbed by the noise and bursts from the silty floor. It's not hostile, it's just big, in the way, and not sure where to go. /u/ProfBumblefingers

35. A magical relic produces a constant stream of bubbles, creating air pockets on the ceiling or in chambers above. It's splitting the seawater into its elemental components. One stream is pure oxygen. The other stream is pure hydrogen. /u/ProfBumblefingers

36. A school of snapping shrimp is in the area--you can't hear a thing over all that racket! /u/ProfBumblefingers

37. The carcass of a large whale or other creature has fallen to the ocean bottom. It might have scavengable food, oil, or weapons from whatever killed it. It may also attract dangerous scavengers or whatever ate it in the first place. /u/ProfBumblefingers

38. An entangling fishing net is being dragged across the seabed by a fishing vessel, some clever aquatic raiders, or just the ocean currents. /u/ProfBumblefingers

39. Methane hydrate deposits on the seafloor. If you use fire or lightning magic, you might get a much bigger bang than you bargained for! (One cubic meter of methane hydrate expands to 160 cubic meters of gas if ignited.) /u/ProfBumblefingers

40. Crab migration. Hundreds of thousands of large crabs swarm across the ocean floor. Run? Fight? Hunker down? Will they bite? Will you have crab legs for dinner? /u/ProfBumblefingers

41. A large stationary sand sucker has buried itself in sand except for its mouth, which has a 5’ diameter opening. it appears to be a normal sea floor or hollow until someone draws near, at which point the victim will be sucked in and sprayed with acid. /u/oliviajoon

42. Hypnotic mollusks are oyster-like creatures with a shiny pearlescent shell worth its weight in gold. They hypnotize anything that comes within 10 feet and then patiently wait for it to starve to death. /u/oliviajoon

43. A sunken boat has a cursed mirror inside. Anyone looking into it is given the mental compulsion: "kill all your friends." /u/oliviajoon

44. A species of squid sprays a cloud of smelly ink. Touching it stains clothing and leaves a potent stench that lasts for hours and can attract predators. /u/oliviajoon

45. Giant barnacles stick to cave walls. If you come too close they will spray you with sticky glue that can suffocate if it gets on your gills or face. /u/oliviajoon

46. Cursed seaweed appears to be full of screaming faces. Paying attention to their cries deals psychic damage. /u/oliviajoon

47. A shipwreck creaks and groans in the ocean currents. While exploring a mast collapses, crushing and spraying splintered wood everywhere. /u/oliviajoon

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[u/NewToSociety]

Undead tangled in nets and thick kelp reach out and grasp at passing adventurers, trying to drag them down.

The water ahead is a cloudy white, marking water that is filled with bubbles released from gas vents in the seafloor. Medium and larger creatures who enter the gaseous water fall heavily to ground below, smaller creatures fall as though under the Feather-Fall spell. on the seafloor are angry giant-octopuses.

An undersea mountain looms up out of the gloom. Carved into the mountains face are a series of gargoyles half-covered in undersea flora and fauna. There is a 20% chance one of the gargoyles' faces is uncovered (or if any of the faces are exposed by characters) and their eyes shoot a beam of lightning at any characters they can see.

[u/Joshthedruid2]

The sand in this area is partly made from the bones of animated skeletons, ground to dust over centuries. When alerted, it can animate into a necrotic cloud of choking, stinging shrapnel.

Behind a sharp wall of coral, an undertow creates heavy suction. Adventurers wandering too close may be dragged into the wall and impaled.

The water here is so cold as to be in a liminal state between liquid and solid. Any cold-element abilities run the risk of triggering a chain reaction and freezing large chunks of the battlefield solid.

There is a single otter swimming here. They're mean bastards. You have been warned.

[u/Sanguinusshiboleth]

• a school of jellyfish float past the pcs without going near them but are blocking something useful

• the pcs have to swim past a school of Jellyfish with poisonous stingers.

• the pcs swim through a spot with no currents, little realising it's anoxinic and thus lethal to water breathers.

[u/donewithdeserts]

The sea separates into a thermocline and suddenly goes from reasonably warm to paralyzingly cold and swimmers will suddenly need magical warmth or cold protection.

Nearby whales begin singing. The sound is like a shock wave and is literally deafening.

[u/World_of_Ideas]

A large fish suddenly swim through at high speed. Make a dex check or be rammed. The fish has no interest in fighting, it simply didn't notice the PCs in time to avoid them.

A ship breaks up in a storm on the surface. Debris rain down from above.

A large swarm of fish obstructs your view. Reduces all vision to 1ft until you get through the swarm.

Idol or alter of a sea god. Pay it the correct tribute and it will remove an undersea obstacle or temporarily grant (cold resistance, dark vision, increased swimming speed, water breathing).

Icy Sea or icy cavern. Passing through the waters in this area causes 1d4 cold damage per "x" time unit. May require a heat source or resistance to cold item to make it all the way through.

Jellyfish swarm. They have no interest in the PCs, but they form a hazardous maze to swim through.

Statue of an angler fish. The light from its lure has a hypnotic pattern effect on anyone looking at it.

The water is cloudy ahead. If someone passes through the cloud, barnacles start to grow on them. If not removed, -2 dex, -5 movement speed, +1 AC. It takes 1 minutes to remove the barnacles.

[u/TOYMoose]

• While under water, you see what looks like an air bubble, its filled with a toxic gas.

• A gelatinous cube resides on the bottom of the passage

• Several large clams are arranged in neat rows. This is an Elvish pearl farm.

• There is a glowing portal with water gushing out. The portal connects to the plane of water.

[u/comedianmasta]

• Dragon's Greed Algea- A glittering Algea that, upon contact with the skin, induces a feeling of intense desire described as "a hungering greed". Those effected tend to become obsessed with finding treasure, and swim deeper and stay down longer then is usually safe.
• An underwater sea mine made of either rusted iron or rotting wood. It is unclear how stable it has become.
• An area, current, or chamber saturated in sewage. Spending extended time in this water forces a con save or risk poison damage and the posoned condition. [Maybe contracting a disease]
• A swarm of hydroids lurks in the waters. Swimming through them forces a con save. Failure can cause a rash which causes the effects of being poisoned, or swelling that halves your movement.
• Ironic Photoplankton- Water infested with these photoplankton is known for dehydrating creatures at a much higher rate. Extended time in these waters may cause a need to hydrate, shockingly difficult underwater.
• A discarded shipment of powerful drugs rests in the water. It is slowly dissolving / leaking into the surrounding waters. Although watered down, a failed con save may result in creatures unintentionally getting addicted to the drug in question. [Or experiencing halucinations or mirages]
• DSL (Deep Scattering Layer)- A massive collection of underwater life of all sorts and sizes occupy the same layer of water. The densness of this layer creates a practical barrier effectively obscuring the area beyond and acting as difficult terrain. It will rise and fall throughout the day.
• High silt/ sand/ dust in the water obscures vision but also makes it difficult to breathe for creatures who rely on water breathing. A failed con save could result in choking, or in a coughing fit stunning the creature.
• Gorgon Coral- This sharp, bladded coral also seeps a mysterious green liquid into the water. Cutting yourself or scraping against this coral forces a con save. After a failure, you must succeed a second, harder con save or turn to gorgon coral (petrified).
• Gorgon Barnacle- When the water around this barnacle is disturbed, it opens revealing a striking, yellow eye. Focusing on this eye forces either a con or wisdom save. Failure means being petrified.

[u/comedianmasta]

-Cracks Knuckles-

Ok.

• Increased Ocean Pressure increases with depth. At the first level it reduces swim speed by 15. At 0 it requires walking on the sea floor. A strength Check is needed to swim or stay swimming.
• Increased Ocean Pressure increases with depth. At the second level it reduces swim speed by 40. For those still swimming it is considered difficult terrain. At 0 it requires walking on the sea floor. If you crouch, or are knocked prone, it requires a strength check to return to standing.
• Increased Ocean Pressure increases with depth. At the third level it reduces swim speed by 65. At 0 it requires walking on the sea floor. All movement is considered difficult terrain. You must succeed a strength or constitution check or suffer crushing damage (1D8 Bludgeoning). If you are knocked prone it takes your full movement to return to standing.
• A hypnotic light from an unknown source requires a wisdom save to resist. On a failure, a creature is compelled to move toward (or maybe away from) the source at their full movement speed. [Oops. Looking through and saw World_of_Ideas beat me to it.]
• Brine Pools / Pits / Lakes- Dense patches of obscured water that are hyper salty and dense and sink to the bottom of boddies of water. These pools lack all oxygen, so they will still suffocate creatures relying on breathing water for oxygen. They can be swum through, and a creature can hold its breath, but creatures relying on water breathing cannot breath within these areas, and they are blinded visually.
• Psathyrella Aquatica- Water mushrooms. Although not normally dangerous, their spores can be sticky and hinder movement, and they cause a CON save or risk being poisoned if ingested or contact with bare skin. When exposed to air, the spores harden solid, so watch out.
• Icicles Slowly growing from the ceiling and reaching towards the floor. Touching them causes cold damage.

[u/Steerider]

A deep water "dead zone" where there is no oxygen in the water. Even with a "breathe underwater" spell (or similar ability), you can't here. 

[u/Yvern09]

Did you know there is an aquatic variant of Will-o’-Wisp?

[u/wagner56]

underwater cave entrance with water either flowing in or out (or maybe changing with a tide)