1. Pertaining to the depths of the ocean to which light does not penetrate; unfathomable: In order to explore the abyssal regions of the sea, specialized underwater robots were used.
2. In oceanography, of or relating to the deepest regions of the sea and to the organisms inhabiting that environment at depths between 4,000 and 6,000 meters (13,123 feet and 19,685 feet): The abyssal fishes appeared to be blind because there was no light where they lived.

A flat, sediment-covered area in the deep ocean basin, usually at a depth between 3,000 and 5,000 meters: Several sunken ships have been mired in the abyssal plains of the Atlantic Ocean.

In oceanography, of or relating to the deepest regions of the sea and the organisms inhabiting that environment: There was an article in Jim's newspaper reporting that a ship recently sank in the abyssal-benthic area of the Pacific Ocean near California and was completely inaccessible or unreachable.