The fish and other creatures living in the hadal zone include viperfish, jellyfish, tube worms and sea cucumbers. These animals have special adaptations that allow them to survive in the completely dark, highly pressurized environment.
The hadal zone is the deepest oceanic region. Its upper boundary lies 20,000 feet below sea level, and its deepest known point is more than 35,000 feet beneath the surface. Most of the hadal zone is extremely cold because it receives no sunlight. However, thermal vents along the sea floor create warm pockets conducive to aquatic life.
The fish living in or near the hadal zone look far different than those living in shallower locations. For example, many denizens of the deep are bioluminescent. Chemical reactions in certain organs produce bright light that attracts curious prey and intimidates predators.
Other deep ocean fish are blind, and most have drab appearances devoid of the festive colors and bold markings found on other fish. Many fish living in the hadal zone are transparent or nearly so.
The hadal zone remains one of the planet's most mysterious environments because the stratospheric pressures are dangerous to humans and exploratory vehicles. For example, vessels that descend to the bottom of the Challenger Deep, the part of the hadal zone, must endure pressures 1,100 times greater than on the surface.