| D20 | Adventure Hook |
| 1 | A large and biodiverse coral reef is inexplicably dying—though the recent arrival of a controversial pharmaceutical company just up-current from the reef draws the suspicions of the planet’s inhabitants, who look to outsiders to investigate. |
| 2 | A deep-sea fishing vessel recently discovered the underwater wreckage of a massive, interplanetary research starship. The starship in question disappeared without a trace years ago amidst controversy over the ethics of its research. |
| 3 | Unusual geothermal activity has been reported near an underwater settlement. Scalding water spouts from small holes with unnaturally even spacing, stretching for miles and appearing to form a seam on the ocean floor. Are they relics of a past civilization, an incursion from deep below the world’s crust, or something even stranger? |
| 4 | A planet’s kelp forests are a hot spot for tourists, but rangers charged with protecting the natural resource have become perplexed by a wave of recent reports of nonaquatic creatures attacking visitors among the forest’s deceptive, twisting strands of kelp. |
| 5 | Each year, a small settlement hosts a months-long festival of lights during the planet’s bioluminescent algal bloom. This year, during the peak of the festival, all the algae instead began to exude an ominous, purple-black light, pulsing together to an unheard rhythm. |
| 6 | The PCs win a stay at a deep-sea luxury hotel. Their suite is a large pod with 360-degree views of the artificially lit ocean floor. Their first night there, the lights flicker and go out, immediately followed by the first of many distant screams. |
| 7 | While conducting seismic imaging in search of natural resource deposits, scientists are shocked to discover two massive, inverted pyramids buried beneath the ocean floor. |
| 8 | A rare disease leaves many of an archipelago nation’s children in critical condition. The only known cure is the nectar of a flower that blooms only in the deepest chambers of aquatic caves guarded by territorial sea-born predators. |
| 9 | Amidst a prolonged famine, officials introduce a new, edible species of sea slug genetically modified to adapt to unfavorable conditions. The slugs turn out to be a little too adaptable, and they begin feasting on the town’s inhabitants. |
| 10 | Magical, spherical pockets of oxygen-rich atmosphere have begun springing up randomly, with devastating consequences for the aquatic flora, fauna, and sapient creatures. Did this phenomenon result from an incursion of the Elemental Plane of Air or perhaps a failed magical or technological experiment? |
| 11 | A large, drifting city suddenly pulls away from its projected trajectory by a mysterious new current. Reports from nearby cities indicate they’re all spiraling toward one central location of concentrated magical energy. |
| 12 | A small research station located on the perimeter of Reaper’s Basin, a deep-sea methane lake, has sent out a distress signal. At the station, the researchers have vanished with all that remains being their uniforms neatly folded on chairs around a table set with still-warm food. |
| 13 | The massive domes protecting a sprawling city on the sea floor have inexplicably begun to crack, sending mass panic through the air-breathing populace. |
| 14 | A film crew working on an action vid blockbuster is woefully underprepared for shooting deep underwater. To make matters worse, what equipment they do have is being methodically sabotaged, putting dozens of lives at risk. |
| 15 | Locals whisper of an iridescent ghost ship that appears on moonless nights over the wreckage of a sunken merchant vessel. When it appears, screams issue from the vessel before it rends in two and sinks beneath the waves. |
| 16 | Communication with an underwater city has been cut off, and the only access to the trapped population is a network of massive elevators and platforms initially built to transport equipment for the city’s construction. |
| 17 | An island known for its picturesque, sandy beaches has become a stinking isle of rotting fish and marine creatures. It seems something in the water has caused the creatures to beach themselves out of desperation. |
| 18 | For the past month, seaweed-draped skeletons have marched from the ocean each night to terrorize a coastal town. Rumors connect the attacks to a necromancer who was exiled from the town years ago. |
| 19 | A submarine cruise liner has abruptly ceased communication with the surface world. Analysis of its final messages seems to indicate that its crew and guests have become zealous initiates of a dangerous cult. |
| 20 | A gargantuan deep-sea creature has absconded with an entire undersea research station that contains invaluable data and equipment. Retrieving these valuables will require navigating the creature’s cavernous lair filled with parasitic threats as well as finding a way to safely extricate the building itself or the resources within from the belly of the beast. |