Abyssal (Adjective)

Relating to ocean depths from 2000 to 5000 meters.

Relational adjectives (pertainyms).

The abyssal plain stretched out before the deep-sea submersible like an endless desert.
Abyssal fish have evolved to survive in the harsh conditions of the deep ocean, where the pressure is extreme and the light is scarce.
The scientists were thrilled to discover a vast abyssal trench, teeming with strange and exotic creatures that had never been seen before.
The warm water of the abyssal thermal vents created a unique ecosystem, supporting a community of organisms that relied on the chemical energy.
Abyssal sediments, such as those found in the Mariana Trench, can provide valuable insights into the Earth's geological history, including information about ocean currents and climate change.

Resembling an abyss in depth; so deep as to be unmeasurable; "the abyssal depths of the ocean".