It's called the Benthic Zone, though this name can apply to deep lakes as well as oceans.
Conglomerate rock
abyssal plain
seamont ?
continental margin deep-ocean basin
benthic realm
The plain on the ocean floor just south of Australia is called the Southeast Indian Ocean Basin.
oceanographer
Sea floor spreading
Wreck debris or a debris field
Challenger Deep.
The Marianas trench .
An ancient under sea valley carved in the ocean floor is known as a canyon, usually given the name of the contributory river on the land. such as the Hudson Canyon, or the Clutha Canyon.