40+ Deep Sea Features For Fantasy Worldbuilding29 min read

Rifts, ridges, vents, and seeps! Welcome back, Outlander, to the 13th entry in Mythic Ecology, my series on how learning real-world landscape features can enrich our fantasy worldbuilding and storytelling. In this post I return to my minimalist framework for Dungeon Masters, Game Masters, fiction writers, and similar worldbuilders to merge the realms of general myth and geomorphology. Last entry I took a look at caves. As I resume my journey sketching a framework for designing Yridia, my unique D&D 5e fantasy world, let’s learn some Deep Sea terms, with a visual guide!

All the images herein I use for educational and entertainment purposes, I claim no rights to any of them. For corrections or content removal requests, hit my contact page.

Let’s revisit my minimalist framework for my worldbuilding. The six archetype tags with which I will flag all the various real-world land features in my Mythic Ecology Series:

1. Settlements: habitable regions of either Work or Play, Familiar or Exotic, offering diverse narrative functions: a Day in the Life, Home Base, Personal Reasons, Gathering Supplies. Can subvert tropes with Ruins or Escape.
2. Omens: sensational, temporal, or particularly pointed features that offer narrative functions of forshadowing, and good or evil portents. Can subvert tropes with a Wild Goose Chase.
3. Overlooks: sites of magnitude and grandeur, living monuments which can function narratively for finding resolve, invoking spirits, or as a Call to Adventure. Can subvert tropes with Dread or Betrayal.
4. Passageways: transitional journeylands, including magical portals, functioning narratively for initiation and return, thresholds and tests, shortcuts and setbacks.
5. Abyss: a void or confined space presenting scarcity or temptation, desperation and danger. Can subvert tropes with a Timely Rescue or Secret Refuge.
6. Battlegrounds: sites fit for epic, sprawling encounters and climax conflicts. Can subvert tropes with Alternative Solutions.

Feel free to submit your own ideas, or draw outside the lines. Alright, let’s see how deep sea features fit in.

The zones below collectively comprise the Aphotic Zones, sea layers without sunlight, which for our purposes constitutes the “Deep Sea”.

Middle Pelagic Zone (Twilight Zone) – a dynamic ocean zone with large temperature variations, with between 1% of incident light and zero light. Distance below ocean surface: 600-3300 feet (200-1000 meters). Bioluminescence adaptation common at this depth and below. Example species: bioluminescent jellyfish, blobfish, bristlemouths, and giant squid.
[Overlooks, Passageways, Abyss, Battlegrounds]

Bathyl Zone (Midnight Zone) – a cold, pitch black ocean layer which sunlight cannot reach, devoid of plants. Distance below ocean surface: 3300-13,000 feet (1000-4000 meters). May feature whale falls & jelly falls. Bioluminescent adaptation extremely common. Example species: dumbo octopi and giant squid.
[Overlooks, Passageways, Abyss, Battlegrounds]

Abyssal Zone (Lower Midnight Zone) – a very cold, high pressure layer of the ocean in perpetual darkness, the bottom of which has no oxygen and holds high concentrations of nutrient salts, as dead organic matter drifts down and decomposes (called “marine snow” and “sea snot”). Distance below ocean surface: 13,000 feet (4000 meters) to above the ocean floor. Supports very few creatures, often transparent and eyeless. Example species: basket stars, sea pigs, sea spiders, and small squid.
[Overlooks, Passageways, Abyss, Battlegrounds]

Hadal Zone (Ultra-Abyssal Zone) – the deepest part of the ocean, lying within oceanic trenches, consisting of long and narrow v-shaped depressions, at extreme pressures. Distance below ocean surface: 20,000-21,300 feet (6000-6500 meters). Supports extremophiles and limited other species adapted to the harshest thermal, chemical, and pressure conditions. Example species: carapace-less crustaceans, cusk-eels, sea anemones, and snailfish.
[Overlooks, Passageways, Abyss, Battlegrounds]

Organisms surviving in the Deep Sea often exhibit many rare traits and adaptations, including the following.

Chemosynthesis – some organisms have adapted to subsist of off chemicals, such as hydrogen sulfide, instead of sunlight (e.g. giant white clams).
Potential D&D 5e Monster: Merrow.

Divergent Coloration –
1. transparency (e.g. jellies & squids),
2. black (e.g. blacksmelt fish),
3. red (e.g. shrimp & squids),
4. reflectivity (e.g. hatchetfish),
5. bioluminescence (e.g. angler fish).
Potential D&D 5e Monster: Tarrasque.

Gigantism – colossal size (giant squid, giant osopod, king-of-herrings oarfish).
Potential D&D 5e Monster: Kraken.

Long Lifespan – some organisms at these depths have superior metabolism, homeostasis, immune system, or lack of predators (e.g. rattails, orange roughy).
Potential D&D 5e Monster: Aboleth.

Pronounced Mouth & Teeth – species who can swallow creatures larger than themselves in one bite, or who have giant fangs (e.g. gulpers, fangtooth fish).
Potential D&D 5e Monster: Giant Shark.

Rapid Growth – accelerated bodily development. (e.g. giant tube worms).
Potential D&D 5e Monster: Hydra.

Deep sea protrusion landforms like Seamounts can support dense populations compared to neighboring areas, due to nutrient-rich waters sliding up from the ocean floor. Includes species like cold-water corals, sponges, sea anemones, and sea fans.

Abyssal Fan / Deep Sea Fan / Underwater Delta / Submarine Fan – underwater geological structures associated with large-scale sediment deposition, similar to alluvial fans. Formed by turbidity currents, underwater landslides along continental shelves.
[Omens, Overlooks, Passageways, Battlegrounds]

Abyssal Hill – a small hill that rises from the floor of an abyssal plain, with sharp edges. Extremely common.
[Settlements, Overlooks, Abyss, Battlegrounds]

Oceanic Ridge / Mid-Ocean Ridge / Rise – an underwater mountain system formed by plate tectonic spreading, as magma swells and dries.
[Settlements, Overlooks, Battlegrounds]

Seamount – a mountain rising from the ocean seafloor that does not reach sea level; often an extinct volcano.
[Settlements, Omens, Passageways, Abyss]

Aseismic Ridge – a chain of seamounts under the ocean created by a hotspot under the Earth’s crust.
[Settlements, Omens, Passageways, Abyss]

Tablemount / Guyot – a seamount with a flat, eroded summit which has subsided and sunk below sea level.
[Settlements, Omens, Passageways, Abyss]

Abyssal Plain – a large, flat, smooth area on the deep ocean floor. Caused by tectonic spreading of seafloor and lower oceanic crust melt.
[Settlements, Passageways, Abyss, Battlegrounds]

Oceanic Basin – large geologic basins below sea level.
[Settlements, Passageways, Abyss, Battlegrounds]

Oceanic Plateau / Submarine Plateau – a relatively flat submarine region rising well above ambient seabed level, with one or more relatively steep sides. Produced by volcanic activity.
[Settlements, Passageways, Abyss, Battlegrounds]

Oceanic Gap / Rift – a steep-sided opening through a submarine mountain ridge.
[Settlements, Omens, Overlooks, Passageways, Abyss]

Oceanic Trench – long and narrow depressions of the sea floor; the deepest part of the ocean floor. Appear among convergent plate margins, varying with sediment supply, accretionary prisms, lithosphere age, and uneven slab motion.
[Settlements, Omens, Overlooks, Passageways, Abyss, Battlegrounds]

Oceanic Trough – a gently sloping depression in the ocean floor shallower, shorter, narrower, and topographically gentler than oceanic trenches.
[Settlements, Omens, Abyss, Battlegrounds]

Submarine Canyon – a steep-sided valley cut into the seabed of the continental slope, serving as channels for turbidity currents across the seafloor. Formed by turbidity current erosion and the slumping and mass wasting of continental slope.
[Settlements, Overlooks, Passageways, Abyss, Battlegrounds]

Hydrothermal Vent – a seafloor fissure issuing geothermally-heated water too pressurized to boil, which can form from rock and mineral ore deposits. Associated with volcanic activity, tectonic spread, ocean basins, and hotspots. Highly-adapted extremophile organisms may subsist here, subsisting independent of sunlight via chemosynthesis.
[Settlements, Omens, Passageways, Abyss]

Black Smoker – a seabed hydrothermal vent appearing as black, chimney-like structures emitting hot clouds of black sulfides. Formed as superheated water beneath the crust rises through the ocean floor, with dissolved minerals reacting with the colder ocean water above.
[Settlements, Omens, Passageways, Abyss]

White Smoker – a seabed hydrothermal vent emitting lighter-hued minerals such as barium, calcium, and silicon, generally with lower-temperature plumes than black smokers due to greater distance from their heat source.
[Settlements, Omens, Passageways, Abyss]

Cold Seep / Cold Vent – an ocean floor area with seepage of hydrocarbon-rich fluids like hydrogen sulfide and methane, appearing like lakes, often in the form of a brine pool. Cooler than the neighboring hydrothermal vents but warmer than the surrounding sea water. Supports endemic species among unique carbonate rock formations and reefs.
[Settlements, Omens, Passageways, Abyss]

Brine Pool – large craters full of brine on the ocean basin, formed via salt tectonics.
[Settlements, Omens, Passageways, Abyss]

Clathrate Hydrates – crystalline water-based solids which trap certain gases or polar molecules in hydrogen-bonded ice, potentially leading to seeps of greenhouse gases like methane.
[Settlements, Omens, Passageways, Abyss]

Petroleum Seep – a consistent outflow of natural liquid or gaseous hydrocarbons, escaping along geological layers, or across them through fractures and fissures in the rock, or directly from an outcrop of oil-bearing rock. Results from reservoir seal breach by various types of fluid expansion or rapid sedimentation, combining with overpressure and buoyancy effects.
[Settlements, Omens, Passageways, Abyss]

Pockmarks – seabed craters caused by fluids erupting and streaming through sediment.
[Settlements, Omens, Overlooks, Passageways, Abyss, Battlegrounds]

Submarine Mud Volcanoes – undersea landforms created by the eruption of mud or slurries, water and gases, as hot water blends with subterranean mineral deposits, without magma.
[Settlements, Omens, Passageways, Abyss]

Types of organisms specially adapted to aggregating together in the harsh environments of seamounts, hydrothermal vents, and cold seeps, plus exotic localized habitats.

Bacterial Mats – mats of bacteria forming around the hydrogen sulfide of hydrothermal vents.
[Settlements, Omens, Passageways]

Clam Beds – beds of giant white clams subsisting off the hydrogen sulfide of hydrothermal vents.
[Settlements, Omens, Passageways]

Crab Mounds – near hot, mineral-rich hydrothermal vents, swarms of crabs can form living mounds.
[Settlements, Omens, Passageways]

Deepwater Coral / Cold-Water Coral – mounds of corals which can extend down in the abyssal zone, clustering on hard or raised deep sea landforms into patches, banks, piles, thickets, or groves to form localized habitats, such as near hydrothermal vents.
[Settlements, Omens, Passageways]

Eel Seethings – eels adapted to survive in groups in deep sea volcanic craters, some of the only vertebrates known in the area.
[Settlements, Omens]

Mussels Mounds – masses of deep sea mussels which cooperate to form mounds around hydrothermal vents.
[Settlements, Omens]

Sea Anemone Clusters – clusters of aggregating sea anemones.
[Settlements, Omens]

Shrimp Swarms – swarms of shrimp with symbiotic relations to chemosynthetic bacteria which allow them to subsist on hydrogen sulfide near hydrothermal vents.
[Settlements, Omens]

Stalked Barnacle Clusters – clusters of stalked barnacles near hydrothermal vents.
[Settlements, Omens]

Tubeworm Bushes – masses of tubeworms, creatures who need no oxygen and have no digestive systems, but which may grow up to 10 feet long (~3 meters). Capable of chemosynthesis to subsist upon cold seeps using symbiotic bacteria, they root into sediment to absorb sulfides in ways resembling plant bushes.
[Settlements, Omens]

Tubeworm Fields – masses of tubeworms, creatures who need no oxygen and have no digestive systems, but which may grow long. Capable of chemosynthesis to subsist upon cold seeps using symbiotic bacteria, they can root into sediment to absorb sulfides in ways resembling fields of grass.
[Settlements, Omens, Passageways]

Whale Falls & Jelly Falls – complex localized ecosystems formed as the carcasses of whales or jellies fall into the Bathyl or Abyssal Zones of the ocean floor. Due to the cold temperatures, high pressure, and lack of oxygen in these zones, this detritus can persist for decades instead of being consumed quickly by scavengers, as in shallower waters.
[Settlements, Omens, Passageways]

Deep Sea Volcanism – submarine volcanoes and magma activity, with either slow or fast eruptions. Associated with hotspots, hydrothermal vents, and tectonic motion, often located near mid-ocean ridges.
[Omens, Passageways, Abyss]

Lava Arches – deep sea lava solids which resemble ruins. Periodically shaken by oceanic earthquakes.
[Settlements, Omens, Overlooks, Passageways, Abyss, Battlegrounds]

Pillow Lava – pillow-shaped structures, primarily of basaltic composition but sometimes others, arising from the extrusion of lava underwater.
[Omens, Abyss]

Seafloor Lava Flow – underwater lava fields comprised of new, darker lava and older, lighter, sedimented seafloor, indicating submarine volcanism beneath the seafloor.
[Omens, Passageways, Abyss, Battlegrounds]

Submarine Earthquake – an earthquake at the bottom of the ocean, often leading to tsunamis. Arises due to plate tectonics.
[Omens, Abyss, Battlegrounds]

Tsunami – a series of enormous waves created by an underwater disturbance such as an earthquake, landslide, volcanic eruption, or meteorite. Waves travel outward in all directions from the origin, building in height as they approach the shore. Succeeding waves can increase in size.
[Omens, Abyss, Battlegrounds]

I hope you enjoyed this thirteenth entry in my Mythic Ecology series! I look forward to continuing with it, I have some greater ambitions for developing this series into worldbuilding web tools. Give this a share if you liked it, and let me know in the comments if you have any feedback. I publish new posts on alternating Tuesdays. In the meantime, I post original D&D memes and writing updates over on my site’s Facebook Page. Also, if you want to keep up-to-date on all my posts, check out my Newsletter Sign-Up to receive email notifications when I release new posts. A big thanks as always to my Patrons on Patreon, helping keep this project going: Adam, Alexander, Anthony, Benjamin, Chris, Eric & Jones, Evan, Geoff, Jason, Rudy, and Tom. Thanks for your support!