One of five vertical ecological zones into which the deep sea is sometimes divided. This is the zone starting from 100 to 700 m deep (coinciding with the upper limit of the psychrosphere) at the 10 C isotherm. The number of species and populations decreases greatly as one proceeds into the bathypelagic zone where there is no light source other than bioluminescence, temperature is uniformly low, and pressures are great. This overlies the abyssopelagic zone and is overlain by the mesopelagic zone.

This is the lowest of the three vertical sections of the pelagic part of the ocean, the other two being the upper euphotic and the middle mesopelagic.