What lives at the bottom of the ocean? To answer that, it helps to see how much ocean there is down there. 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. People have explored only a small part of the deep ocean. As of April 2026, only about 28.7 percent of the ocean floor had been mapped in detail, and scientists think most kinds of ocean life have not yet been discovered or named.

Here are the layers of the deep ocean, from the top down. The twilight zone is the layer from about 200 to 1,000 meters (656 to 3,280 feet) down, and only a little light reaches it. Below 1,000 meters is the midnight zone, which reaches down to about 4,000 meters. It has no sunlight at all and stays near 4 degrees Celsius. Below the midnight zone is the abyss. It is cold and dark with crushing pressure, yet animals and microbes live there. As of November 2026, the average ocean is about 3,682 meters (12,080 feet) deep.

The hadal zone is the deepest part of the ocean. It is found in deep trenches, below about 6,000 meters (19,700 feet). The deepest place in the ocean is called Challenger Deep, and it is in the Pacific Ocean. NOAA gives its depth in November 2026 as about 10,935 meters (35,876 feet). The Mariana Trench is a long, deep trench in the floor of the western Pacific Ocean, near Guam. In 2012, the filmmaker James Cameron dived alone in a one-person submarine, Deepsea Challenger, to the bottom of Challenger Deep.

Three conditions shape life in the deep. The first is darkness: far down in the deep ocean it is dark, because sunlight does not reach there. The second is cold. The deep ocean is very cold, about 4 degrees Celsius (39 degrees Fahrenheit) on average. The third is pressure. 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.

You might guess that nothing makes light in a place this dark. In fact, many animals in the deep sea make their own light. Making light like this is called bioluminescence. NOAA says about 80 percent of the animals living between 200 and 1,000 meters down are bioluminescent. 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 that lure holds bacteria that make light.

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 of the deep sea is hot. A hydrothermal vent is a place on the sea floor where very hot water comes out of the ground. 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 vent communities in 1977, and 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. This is called chemosynthesis. Giant tube worms live at vents and 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.

Reaching these layers takes special machines. In 1960, Jacques Piccard and Don Walsh were the first people to go to the bottom of Challenger Deep, in a deep-diving craft called Trieste. Alvin is a research submarine that carries up to three people to the deep sea floor. It was commissioned in 1964. Scientists also use robot submarines called ROVs, steered from a ship, to explore the deep ocean. As of November 2026, NOAA Ship Okeanos Explorer is described as the only federal vessel dedicated to exploring our largely unknown ocean.

You may have heard that nothing could live at the very bottom. In fact, 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. Giant squid can be up to about 13 meters (43 feet) long, and a giant squid has eyes as big as dinner plates. In 2012, scientists from Japan filmed a giant squid alive in its deep-sea home for the first time. What else is down there, in the dark?

Words to know

• bioluminescence: light from animals
• hydrothermal: hot-water
• abyss: deep dark layer
• chemosynthesis: food from chemicals
• pressure: pushing force