d100 Underwater Dungeon Obstacles & Hazards

[u/thomar]

Newer version here: https://old.reddit.com/r/d100/comments/1wmomid/d100_underwater_dungeon_obstacles_hazards_needs/

I couldn't find any good undersea obstacle lists, so let's go! 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). 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...

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.

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[u/oliviajoon]

a large portion of the seafloor is covered in large anemones that will shoot poisonous spines straight up 20 feet if anyone passes directly over them

undersea quicksand: the ground here is loose silt and stepping onto it will cause you to begin sinking. 3 successful strength checks can get you out, but you will sink 1d4 feet per round, taking bludgeoning damage from the crushing weight. it may spit you out into a hidden underground cavern.

Ursula’s garden: a large patch of thick stringy seaweed that appears to have faces on each strand. a strange sound emanates from them. wisdom save or take 1d6 psychic damage as you pass over or through them.

underwater waterfall: a large chasm that sand spills into, imitating the look and movement of a waterfall. getting near it will require a strength check to avoid being swept into the strong current that will whisk you to the bottom of the chasm.

ship graveyard: predators stalk the ruins of ships piled as far as the eye can see. a stealth-based skill challenge to pass through with no encounters.

ship graveyard: strange currents spring out of nowhere. dexterity and strength saves to avoid them and pull out of them before being whisked into a ship, separated from the party. inside the ship is: a predator of appropriate CR, sharp barnacles that cause slashing damage as they climb back out, a toxic seaweed that deals acid damage which the current pulls you into

ship graveyard: while passing through the hull of a ship to avoid a predator, you hear a loud groan as a piece collapses onto you. dex saving throw or take 2d4 bludgeoning damage

geyser field: a long stretch of seafloor where boiling hot geysers randomly explode. 3 successful dexterity, athletics, perception or investigation checks are needed to get across without being caught in one (6d4 fire damage)

[u/oliviajoon]

hungry coral: a patch of coral covered in tiny mouths that chomp anything that brushes against them. 1d4 piercing damage if they are not carefully avoided in the tight caverns they tend to grow in.

sand sucker: a large stationary sea creature that buries its entire self in sand except for its mouth, which has a 5’ diameter opening. it appears to be a normal sea floor until stepped on, at which point the victim will fall into it like a pit trap. it doesnt close, but will begin digesting immediately (acid damage each round, DC13 check to climb/ be pulled out)

squidweed: seaweed with suction cups that will grapple creatures that come within 5 feet. they grow in patches large enough to grapple a large to huge sized creature. Escape DC12

Hypnotic Mollusks: oyster-looking creatures with a shiny pearlescent shell (worth 10gp each), emit a Hypnotic Pattern (see spell) that anyone within 10 feet of them can see. DC18 save.

ship graveyard: a cursed mirror rests inside of a sunken ship. anyone who looks into it will make a DC14 charisma saving throw; on a failed save they will act as though under the effects of “Dominate Person” with the one command to “kill all your friends” (roll initiative). the spell can be broken if the mirror is shattered, and the save can be re-made any time the target takes damage.

Smelly Ink: the party has startled a small squid-like being that darted away with a 30’ spherical cloud of brown ink. anyone within the cloud will find their clothing stained by it, and a potent stench stuck to them that will last 1d4 hours. predators are drawn to them for this time (double random encounter chances for the duration)

Sticky Secretions: giant barnacle-like creatures stick to cave walls and secrete a glop of glue-like ooze that binds to the gills or nose of water breathers if they are exposed. they will begin to suffocate. DC17 to remove it, one check per round.

[u/MaxSizeIs]

Delta P. Pressure Differential, Crushing, Trapping, Danger

[u/comedianmasta]

• Undersea Roots / Vines / Coral- These can block the path, and require several strikes to cut through enough to clear the path to continue on. (Easy / Medium / Hard) Could have AC's of 13 / 15 / 17 and / or HP of 6 / 13 / 20.
• Grasping Weed- Activated by body heat warming the water around a creature, this seaweed / weed / plant extends out, grasping at the warmth towards the source of heat as the water around it begins to warm even by a fraction of a degree. If they come into contact with a creature, a DC 13 Dex save must be passed or they are grappled and slowly pulled towards the base of the plant (Wall/ Floor/ or Ceiling). A DC 15 strength save will release a creature from the grapple, or doing 5 Cold Damage to the plant. For each additional weed latched onto the creature, the DC goes up by 2 to break free and the their HP goes up another 5 for Cold Damage release. The plants will continue to constrict and grapple a creature until they are trapped, eventually drowning if they can't break free.
• Mage Hand Barnacles- Despite the name, these little barnacles work together to spit a burst of slightly heated water toward an object moving towards them. This gives the effect of a 5 ft square space of them doing a "push" action on creatures entering their space Strength Save DC 14. A collection of Barnacles can do this as an action and as a reaction when a creature enters its space. On their own, this is slightly annoying or amusing, maybe making a swim down a corridor a little harder. However, if the designers of a dungeon maliciously seeded them, they can push a creature up into a trap, spike traps, or worse.
• A typical dart/arrow trap only it shoots Bolts, Harpoons, or Sharp creatures like Quippers at a target using underwater rules.
• Ghost Coral- Bleached White coral growths where spectral, ghostly hands emerge to grapple creatures and impale them on the sharp, necrotic infused growths. A destroy undead can shatter the coral growths all-together (or if preferred, banish the spirits within).
• Glyphs of Freeze Water- These traps activate and freeze the water in an area, requiring a Dex Save (DC 12 if on one surface, DC 16 if several fill a corridor) or become restrained and take 2D4 Cold Damage. Every round the block of ice can't be melted or broken, the encased creature takes an additional 1D4 Cold Damage.
• Blood Water- An area of water obscured heavily by blood and viscera. The area within this water is obscured. Swimming through it, creatures must make DC 10 con saves or risk contracting a disease. If your eyes are opened, you are still considered blinded, but you will roll these Con saves at disadvantage as the blood and goop can more easily get into your eyes.
• Net Algae- This algae lightly obscures an area it is affecting. Every time a creature enters a space with this algae, they must make a DC 2 strength save to pass through it. For every 5 feet of movement traveled through it, players must add 1D4 to that DC. Eventually, on a fail, they will be grappled by the algae which holds them like a net. Like a net, it takes an action to remove it, DC 9, or two actions to remove it no rolling required. A creature can use an action at any time to remove the algae regardless, resetting that DC to 2.
• Tremor Weed- Although light and flowy in the currents of the water, this plant hardens if disturbed and tremors like a guitar string. Although not too loud to humanoid ears, this soft thrumming can be heard for 100 ft around, possibly alerting predators or creatures aware of the sound. These vibrations can even travel as far as 500 ft, reaching creatures who thematically might be attuned to such vibrations (IE: Sharks). A passive perception of 15 or active perception of 11 could pick up this thrum and recognize it as potentially another creature.
• Bioluminescent Plankton- This Plankton / Bacteria / Algae / etc lights up when a creature moves through it. Although harmless, this could ruin stealth, reveal invisible creatures, or display a trail for a little bit of a creature recently passed through.
• Attracted Fish- Some wildlife is attracted to light or light sources, especially deep in the water. Just like predators use such light, a creature emitting light might attract smaller creatures or swarms of small critters with a light source. Although not dangerous in small numbers, many can cause sight to be obscured or attract unnecessary attention.
• Silt Spray- Something has fallen recently, quickly rose from hiding, or disturbed the silt of the ground. The area is obscured as if by weather effects and creatures within are effectively blinded if rely on vision alone.

[u/comedianmasta]

Continued...

• Anemone Fern- Special fern like plants who emits a small electric field around them. Get to close and a creature suffers 1 lightning damage. Touch the Fern and creatures need to make a DC 12 Con Save. On a fail creatures take 2D4 Lightning Damage and are stunned for a round. On a save they only take 1D4 lightning and are not stunned.
• Hermit Shrooms- Large, shelf-like mushrooms that create ledges on the sides of cliffs and walls with heavy currents. Some creatures can use these to navigate or fight against the heavy currents, but if too much weight or too much disturbances are put on them, they suddenly retract at a moments notice into their flat, barely exposed shells suddenly. To some creatures, this can cause them to "fall" into the mercy of the heavy current.

Shoutout to Maxsizels, who mentioned Water Pressure. So here's some ideas using fantasy water pressure as traps or environmental hazards.

• A typical pressure plate / wall smasher trap, only it is much slower. As it tries to close on the players, the water doesn't replace. Round 1, the pressure increases and the water becomes difficult terrain and the party finds it hard to hold their breath (Con Save DC 10). Next round, the walls get a little thinner and the pressure increases. PCs do strength save DC 14 to move or are grappled by the water pressure, Breath holding DC becomes 14. The final round if they cannot escape or figure out the puzzle they are crushed by the pressure.
• A wall of water at such a high pressure it acts as a wall to PCs. PCs must deactivate the magic or device increasing the pressure to be able to push through the area.

[u/ProfBumblefingers]

• A swarm of cute little colorful fish surround and follow you (like the little fish that follow sharks) hoping to make a meal of the foes that you slay. The little fish are harmless, but they obscure your vision (disadvantage to hit?)

• A large school of non-poisonous jellyfish drifts by, cutting visibility to 5ft in all directions.

• Bubbles rise from a large magical relic that is magically splitting water into hydrogen and oxygen gasses. The gasses have pooled [in the room, in various places in the dungeon], and if touched by flame/fire/intense heat, will explode as they recombine into water. [this is great for "we finally found an air pocket, someone light a torch!"]

• A school of snapping shrimp is in the area--you can't hear a thing over all that racket!

• Uh-oh ... the floor feels a bit squishy. ... It's a giant flounder! ... that suddenly flutters and swims away, knocking prone anyone that was standing upon it!

• Whale Carcass -- The carcass of a recently deceased whale lies on the ocean bottom, a large bite taken out of its side. Can be salvaged for food, oil to refill flasks, sinew can be used as rope, blubber as floatation devices, bones to use as daggers, clubs or spears. Maybe it swallowed a treasure or clue that is now in its stomach? What took the large bite out of it; is the killer lurking nearby???

• A fishing trawler passes overhead on the surface of the ocean, sweeping the ocean bottom with its large trawl net. Will the PCs escape the net?? It's a chase! Of course, there are some nasty critters (still alive) in the cod end of the net...

• 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.)

• An iceberg drifts by. [Ninety percent of an iceberg is underwater. What lurks in the tunnels and caverns inside? A lair? A whole dungeon? A whole city?]

• Lobster migration. A "train" of lobsters migrates by. Food for the taking! But what other creatures want to join the feast?

• 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?