Most of the ocean is completely dark. This is called the aphotic zone. The aphotic zone begins at a depth of 0.9 km (3,000 ft) and continues to the bottom, which usually maxes out at a depth of 5.5 km (18,000 ft), except for ocean trenches, which can be as deep as 11 km (6.8 mi). In the aphotic zone, photosynthesis cannot occur, so the usual foundation of food webs, plants and cyanobacteria, are absent. Instead, animals have to subsist on carcasses that fall from above, other animals, and in some rare cases, bacteria that use chemical energy sources such as sulfides and methane.

The aphotic zone is further broken into two levels, the bathypelagic zone, which extends from around 1 km (3300 ft) below the surface to 4 km (13000 ft) below. Below this is the abyssopelagic zone, also called the abyssal zone, where pressures are extremely high (400 atmospheres and up) and biodiversity drops sharply. Even below the abyssopelagic zone is the hadopelagic zone, which is used to refer to ocean trenches.

Some animals that live in the aphotic zone include the gulper eel, giant squid, smaller squids, anglerfish, vampire squid, and numerous jellyfish. Some of the ugliest and most evil-looking fish in the world are found here, including the viper fish, fangtooth, dragonfish, lizardfish, and many others. These fish often have hinged jaws, black scales, piercing eyes, and extremely sharp teeth. You wouldn't want to run into them in a dark flooded alley. Some of them are named after their superficial similarity to reptiles.

Many of the animals in the aphotic zone are bioluminescent, meaning they can produce their own light in one way or another. This can be used both for navigation and luring small animals into their jaws. The angler fish is probably the most famous of examples, as it has a lure protruding in front of its mouth from a special appendage on its head.

Other fish look not so evil so much as blob-like. These bottom-feeders include the aptly-named blobfish, coffin fish (which walk along the sea floor using leg-like fins), and others. At the bottom one can also find sea cucumbers and giant isopods, foot-long relatives of woodlice.

The deep sea also has numerous unusual and rare sharks, such as the Megamouth Shark, hailed as one of the most impressive marine animal discoveries of the last century.