The hadal zone (named after the Greek god, Hades, god of the Underworld), also known as the hadopelagic zone and trench zone, is the delineation for the deepest trenches in the ocean. This zone is found from a depth of around 6,000 metres (20,000 ft) to the bottom of the ocean. The hadal zone has low density and diversity of marine life.

In 1960, Jacques Piccard and Don Walsh reached the bottom of the Mariana Trench, the deepest known trench on Earth, and observed life. It is believed that most life at this depth is sustained by marine snow or the chemical reactions around thermal vents. The intense pressure and the lack of light create hostile living conditions and few species are adapted to exist here. As no sunlight reaches this layer of the ocean, deep sea creatures have adapted with reduced eyesight, having very large eyes for receiving only bioluminescent flashes. Most of the bottom-dwelling creatures lack any pigmentation, since coloration is not useful in an environment with no light.