A rare word (not if you are a marine biologist), derived from its noun abyss, relates to the deepest depths of the ocean, usually around 4000-6000 metres below the sea surface. The Abyssal ocean also known as the Abyssopelagic Zone, is completely dark and has close to freezing temperatures. Having no sunlight means this oceanic zone is home to some very interesting sea flora and fauna such as giant squid and anglerfish (like in finding nemo)! Very little is known about the abyssal depths of the ocean with only 5% having been explored.
Another meaning of the word Abyssal is ‘incomprehensible’ however the ocean depth definition is used most commonly, and is a lot more interesting.