Far down in the deep ocean it is dark, because sunlight does not reach there, and the deep ocean is very cold, about 4 degrees Celsius (39 degrees Fahrenheit) on average. The deeper you go in the ocean, the harder the water above pushes down on you. Deep-ocean life is under extreme pressure, from about 40 to over 110 times the pressure of Earth's atmosphere. The deep sea makes up over 95 percent of Earth's living space, and it remains largely unexplored. About three-fourths of the area covered by ocean is deep, permanently dark and cold, and people have explored only a small part of the deep ocean. A question follows from all this darkness, cold and pressure: what do its animals eat?

For most of the deep sea, the answer starts at the surface. Marine snow is a shower of bits of food that slowly falls from the upper ocean to the deep, and many deep-sea animals eat it. As marine snow sinks, flakes clump into bigger pieces that fall faster, but the trip to the bottom can still take weeks to years. A vampire squid catches bits of falling marine snow with two long, sticky threads, then pulls them through its arms to eat. Other animals go to the food instead. Each evening many deep-sea fish and tiny animals swim hundreds of meters up toward the surface to feed, then return to the dark by day. It is likely the largest daily migration on the planet.

Some places on the deep sea floor have another source of energy. A hydrothermal vent is a place on the sea floor where very hot water comes out of the ground. Vents form where seawater seeps into cracks near hot rock, heats up and picks up metals, then rises and meets cold ocean water, and the metals harden into chimney-like towers. Vents that spew black iron sulfide are called black smokers, while vents that spew white minerals such as barium, calcium and silicon are called white smokers. Water coming out of some vents has been heated to up to about 400 degrees Celsius (752 degrees Fahrenheit), as of November 2026, hot enough to melt lead.

Scientists first found hydrothermal vent communities in 1977, and they were surprised by how many kinds of living things were there. At vents and some other places on the deep sea floor, tiny microbes make food from chemicals instead of from sunlight, which is called chemosynthesis. Giant tube worms live at deep-sea vents, where hot water comes out of the sea floor, and they can grow almost five feet long. They have no mouth or stomach, and bacteria living inside them make food from vent chemicals for the worm. Scientists think most kinds of ocean life have not yet been discovered or named.

Vents are not the only special places on the sea floor. When a whale sinks to the sea floor at the end of its life, it becomes a whale fall: food and a home for many deep-sea animals. Whale bone is about 60 percent fat by weight, up to 200 times the nutrients usually found at the sea floor, and bone-eating Osedax worms bore in to get it. Some whale falls can support a blanket of 45,000 worms per square meter, the highest animal density in the entire ocean. Brine lakes are super salty pools on the sea floor, with water so dense that it does not mix with the water above, so you can see a surface and even waves.

Down in the dark, many animals also make their own light, and making light like this is called bioluminescence, which first arose roughly 540 million years ago in a group of corals called octocorals. NOAA says about 80 percent of the animals living between 200 and 1,000 meters down are bioluminescent, and deep-sea animals use their light to find food, to find mates and to protect themselves. A deep-sea anglerfish has a glowing lure that dangles in front of its mouth, and the glowing tip of the lure holds bacteria that make light. The vampire squid is a reddish-brown animal that hides in twilight water where red light does not reach, and its arm tips can glow with pulsing blue light.

Animals live even in the deepest trenches. The hadal zone is the deepest part of the ocean, found in deep trenches below about 6,000 meters (19,700 feet). The deepest place in the ocean is called Challenger Deep. Snailfish live deeper than any other fish scientists have found, in the deepest trenches. The deepest fish ever seen alive was a small snailfish filmed at 8,336 meters (27,349 feet) in a trench near Japan in August 2022. The deepest fish ever captured, two P. belyaevi snailfish, came from 8,022 meters (26,319 feet) in the Japan Trench in August 2022. You may have heard that nothing could live in the deepest, darkest, highest-pressure part of the ocean. In fact, these fish do.

Scientists use robot submarines called ROVs, steered from a ship, to explore the deep ocean, and as of November 2026 NOAA Ship Okeanos Explorer carries a two-part remotely operated vehicle that can dive to 6,000 meters. Alvin is a research submarine that carries up to three people to the deep sea floor. As of November 2026, NOAA Ship Okeanos Explorer is described as the only federal vessel dedicated to exploring our largely unknown ocean. As of April 2026, only about 28.7 percent of the ocean floor had been mapped in detail. Seabed 2030 is a project aiming to produce a complete map of the ocean floor by the end of the decade. NOAA says explorers have seen less than 0.001 percent of the deep ocean seafloor, as of April 2026. What else lives down there?

Words to know

• chemosynthesis: food from chemicals
• hydrothermal: hot-water
• bacteria: tiny living things
• marine: of the sea
• smokers: chimney-like vents