Undersea base

Undersea base
A diver swims down to Aquarius, the undersea laboratory on the sea floor off Key Largo, Florida, during NASA's NEEMO 12 training in 2007 · NASA, Public domain, Wikimedia Commons

A cluster of steel modules bolted to the floor of an alien ocean, hundreds of metres down, where no light from the sky arrives. Inside, the air is warm, close and dripping with condensation; outside, the floodlights reach a few metres of black water and the hull groans under the weight above it. The wet room opens straight onto the sea through a moon pool, and every leak is a jet that can cut. Whatever went wrong here went wrong slowly, and the base kept recording.

Light
Dim
Going
Slow

Hear it

Hazards

  • Crushing depth. Every ten metres down adds the weight of a whole atmosphere. A few hundred metres needs a hard suit, a few kilometres a hull, and far below, models say, the water itself turns to ice under the pressure. D&D 5e (House rule): House rule: below the depth its suit or hull is rated for, a creature takes 4d10 Bludgeoning damage at the start of each of its turns, doubled for each further step of the same depth.
  • Leak under pressure. At this depth the sea pushes on every seam with the weight of dozens of atmospheres. A pinhole leak is a jet hard enough to cut skin, a cracked weld floods a module in minutes, and the bulkheads close on their own, whichever side the crew is on. D&D 5e (House rule): House rule: a creature that crosses a jet makes a DC 13 Dexterity saving throw, taking 2d10 Slashing damage on a failure or half as much on a success. A breached module fills in 1d4 minutes; once the water is over their heads, creatures there must swim and hold their breath (see Suffocation). Sealing a jet takes an action and a DC 14 Intelligence check with tinker’s tools.
  • Living under pressure. The wet modules are kept at the pressure of the sea outside, so divers can come and go through the moon pool. A body that lives at that pressure fills with dissolved gas and cannot simply leave: going back to the dry decks or the surface takes days in a decompression chamber, and a shortcut is paid in pain, paralysis or death. D&D 5e (House rule): House rule: a creature that leaves the pressure without decompressing makes a DC 15 Constitution saving throw at the end of each hour for 6 hours. On a failure it gains 1 Exhaustion level; on a failure by 5 or more it also has the Incapacitated condition until it is put back under pressure in a chamber, which ends the effect after 1 hour.
  • Isolation. Months from anyone, with the same few faces. Small grudges grow, sleep goes wrong, and some start hearing things in the static. D&D 5e (House rule): House rule: after each month of isolation, each creature makes a DC 10 Wisdom saving throw; on a failure it has Disadvantage on Wisdom (Insight) checks and Charisma checks with the crew until it gets a week of leave.

What may happen

  1. The hull groans as the water outside shifts, a long sound like old timbers.
  2. A drip from a ceiling weld, one drop a second, steady as a clock.
  3. Past the viewport, the floodlights reach ten metres of water and then nothing at all.
  4. Something long passes the viewport just outside the lights, and the hull hums as it brushes a mast.
  5. The moon pool in the wet room lies open and calm, and wet footprints lead away from it across the deck.
  6. The base’s last log ends with an order to seal every bulkhead, then hours of tapping.
  7. The lights die in one module; the emergency strips glow green, and the water sounds louder.
  8. A bunk still has its blanket thrown back, and a mug on the shelf has grown a skin of mould.
  9. A pressure alarm sounds in a sealed module; the gauge says the room holds the pressure of the sea outside.
  10. The diving lock cycles on its own, and the window of its inner door shows the chamber full of water.
  11. The minisub is still in its dock, battery at a fifth, hatch unlocked.
  12. Pale flakes drift past the viewport without end, sinking slowly into the dark.
  13. The hydrophone picks up a voice saying a crew member’s name, over and over, from somewhere in the water.
  14. When the floodlights switch off, a cloud of blue sparks blooms outside and drifts away.
  15. A corridor is flooded waist-deep with cold water, and the door at its end is still dry behind its glass.
  16. Three knocks on a bulkhead, from the side that is supposed to be flooded.
  17. The carbon dioxide scrubbers read low; the spare cartridges are listed in a store two modules away.
  18. Pale crabs cover the outside of a viewport, picking at the glass.
  19. The base settles a few centimetres into the mud, and everything loose rolls toward one end.
  20. Two divers are still in the saturation chamber, under pressure; one of them raises a hand to the window.
  21. On the monitor, the cable to the surface buoy goes slack, and the surface link drops.
  22. The medical bay’s recompression chamber is sealed, its counter running, and nobody answers the intercom.
  23. Sonar shows a contact circling the base at a fixed distance, a little larger each time it passes.
  24. Condensation runs down every wall; the air tastes of metal and old coffee.
  25. The hydroponics bay has run wild under lights still on, its tomatoes split on the vine.

Adventure hooks

  1. The base’s owners want the data core brought up before the insurers’ divers get there.
  2. A crew member’s brother has paid for the truth about the last tapes, not for a rescue.
  3. The base still sends its hourly all-clear, and the engineer who wrote that routine says she never did.
  4. Two divers are still in saturation down there, and their decompression will take days the base may not have.

Who and what is met

Each is an SRD stat block under another name, as game masters reskin them.

Other places among the stars

Orbital station, Derelict ship, Colony outpost, Precursor ruin, Asteroid field, Gas giant orbit, Deep space relay, Terraforming works, Prison hulk, Research lab, Crashed freighter, Nebula, Neon city, Junktown, Spaceport, War front, Alien hive, Living ship, Lava foundry

This work includes material from the System Reference Document 5.2.1 ("SRD 5.2.1") by Wizards of the Coast LLC, available at https://www.dndbeyond.com/srd. The SRD 5.2.1 is licensed under the Creative Commons Attribution 4.0 International License, available at https://creativecommons.org/licenses/by/4.0/legalcode.

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