Of or belonging to the ocean depths, especially below 2000 m. The abyssopelagic zone is that part of deep lakes, oceans, or seas characterized by specific forms of plankton and nekton which inhabit open water, and the abyssobenthic zone is the bottom of a deep lake, ocean, or sea. An abyssal plain is the deep sea-floor, formed of abyssal deposits, with a gradient of less than 1 in 10 000. The Canada Abyssal Plain, lying between Canada and the Alpha ridge, is the largest of the Arctic sub-basins, with an average depth of 3 658 m. Abyssal hills, which interrupt an abyssal plain, are 50–250 m high.