Oceanic trenches are underwater chasms that can reach 2-4 km or 1.9-2.5 miles below the surrounding seafloor and are the deepest areas of an ocean. Fictional oceanic trenches are mysterious places that often contain sunken ruins, unknown passages, and unknown creatures. These are dark places, where no sunlight can penetrate to and the pressure of the seawater above can crush anything not designed, or evolved, to withstand such pressure. Such trenches are formed by colliding tectonic plates – the trench is formed where one plate sinks below the edge of another plate. Volcanoes may be generated in the upper plate along the interaction boundary, creating volcano island chains on or near one side of the oceanic trench.

In fictional worlds, oceanic trenches can be eerie and remote locations that hide the darkest secrets of the ocean. There may be submarine caves near these regions, and they may be home to massive leviathans or underwater civilizations. There may be long lost shipwrecks within them or perilously perched on the edge of the trench.