Far down in the deep ocean it is dark, because sunlight does not reach there, and the dark reaches across most of the ocean. 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. The deep ocean is also very cold, about 4 degrees Celsius (39 degrees Fahrenheit) on average. You might guess that nothing makes light in a place like that. Something does. People have explored only a small part of the deep ocean.

The light fades with depth, and the layers of the deep ocean show how. The twilight zone is the ocean 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, and it has no sunlight at all. Below the midnight zone is the abyss, which is cold and dark with crushing pressure, yet animals and microbes live there. The hadal zone is the deepest part of the ocean, found in deep trenches below about 6,000 meters (19,700 feet). As of November 2026, the average ocean is about 3,682 meters (12,080 feet) deep. The deepest place in the ocean is called Challenger Deep, and it is in the Pacific Ocean.

Many animals in the deep sea make their own light, and 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. You may have heard that deep-sea animals are all huge sea monsters. In fact, male deep-sea anglerfish are tiny dwarfs, only a few centimeters long, and in some species a male attaches to a female's body and stays.

The vampire squid is another animal that uses light in a quiet way. It 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. 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. A vampire squid catches bits of falling marine snow with two long, sticky threads, then pulls them through its arms to eat.

The twilight zone is crowded with life. The Smithsonian estimates that about 90 percent of the world's fish by weight live in the twilight zone, roughly 10 billion tons of fish, as of November 2026. 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, and it is likely the largest daily migration on the planet. 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.

A giant squid has eyes as big as dinner plates, and giant squid can be up to about 13 meters (43 feet) long. In 2012, scientists from Japan filmed a giant squid alive in its deep-sea home for the first time. In June 2019 a NOAA-supported team filmed a giant squid in the Gulf of Mexico, about 10 to 12 feet long, the first one recorded alive in U.S. waters. You may have heard that nobody has ever seen a giant squid alive. In fact, both of these films caught one.

Light is not the only way to live in the dark. 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. 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. 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. Giant tube worms have no mouth or stomach, because bacteria living inside them make food from vent chemicals for the worm.

Reaching the dark takes special machines. Scientists use robot submarines called ROVs, steered from a ship, to explore the deep ocean. 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, and its depth rating of 6,500 meters lets researchers reach about 99 percent of the ocean floor, as of November 2026. NOAA says explorers have seen less than 0.001 percent of the deep ocean seafloor, as of April 2026. Scientists think most kinds of ocean life have not yet been discovered or named. What else is glowing down there?

Words to know

• bioluminescence: light from animals
• twilight: dim light
• lure: bait
• migration: long trip
• bacteria: tiny living things