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 NOAA says we know less about the ocean floor than we do about the surface of the Moon and Mars. People have explored only a small part of the deep ocean. 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. Deep-ocean life is under extreme pressure, from about 40 to over 110 times the pressure of Earth's atmosphere. Anyone who asks what lives at the bottom of the ocean is asking about most of the living space on Earth.

Start with how deep the bottom is; the deepest place in the ocean is called Challenger Deep, and it is in the Pacific Ocean. The depth of Challenger Deep depends on the measurement: Britannica lists 10,994 meters for a 2011 U.S. survey, NOAA gives about 10,935 meters, and Guinness reports 10,934 meters (+/- 6 m), as of November 2026. 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. The first attempt to sound the Mariana Trench came during the Challenger Expedition (1872 to 1876), and in 1875 a sounding of 8,184 meters was made near its southern end. The hadal zone is the deepest part of the ocean, found in deep trenches below about 6,000 meters (19,700 feet), and zone boundaries differ a little by source: the Smithsonian puts the abyssal zone at 4,000 to 6,000 meters, while Woods Hole gives 3,000 to 6,500 meters.

In 2012, the filmmaker James Cameron dived alone in a one-person submarine, Deepsea Challenger, to the bottom of Challenger Deep, and his descent took 2 hours and 36 minutes. In April and May 2019 Victor Vescovo piloted the submersible Limiting Factor alone to the bottom of Challenger Deep. The 2019 Challenger Deep depths came from pressure readings by two calibrated CTD sensors on the submersible, and single readings ranged from 10,920 to 10,929 meters, with the sensors averaging 10,925 meters (35,843 feet). By 26 June 2020, Victor Vescovo had made the descent to Challenger Deep a record eight times, using the submersible Limiting Factor and three robotic landers. Alvin is a research submarine that carries up to three people to the deep sea floor, and it was commissioned in 1964. Alvin's depth rating of 6,500 meters lets researchers reach about 99 percent of the ocean floor, as of November 2026; scientists also 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, and, as of November 2026, it is described as the only federal vessel dedicated to exploring our largely unknown ocean. As of November 2026, it is at sea for a 2026 Technology Demonstration, November 2 to 16, testing autonomous underwater vehicles near the Hawaiian Islands.

Food is the central problem of the deep. 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. Sediment traps on the ocean floor show that about 815 million tons of carbon reach the ocean floor each year, mostly as marine snow. Below 1,000 meters is the midnight zone, which reaches down to about 4,000 meters, and below it is the abyss, which is cold and dark with crushing pressure, yet animals and microbes live there. Little is known about the animals in the midnight zone, and even less about its microbial life.

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. 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 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. 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. You may have heard that deep-sea animals are all huge sea monsters; in fact, many are small: male deep-sea anglerfish are tiny dwarfs, only a few centimeters long. The vampire squid is a reddish-brown animal that hides in twilight water where red light does not reach.

Scientists first found hydrothermal vent communities in 1977, and were surprised by how many kinds of living things were there. A hydrothermal vent is a place on the sea floor where very hot water comes out of the ground, and 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. 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, which is called chemosynthesis. Giant tube worms have no mouth or stomach, and bacteria living inside them make food from vent chemicals for the worm. Hydrogen sulfide is normally poisonous, but Riftia tubeworms have blood hemoglobin that binds tightly to both oxygen and hydrogen sulfide, carrying them safely to their bacteria.

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. Brine lakes are high in methane, and certain bacteria use it to make energy, so mussels and crabs gather at the edges of a lake to feed. 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. Scientists estimate whale falls exist every 5 to 16 kilometers in the Pacific Ocean, though knowledge of them comes from few ROV and AUV encounters. 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. Osedax worms house bacteria in their root-like parts that use sulfur in whale bones to make energy by chemosynthesis, and some whale falls can support a blanket of 45,000 worms per square meter, the highest animal density in the entire ocean.

You may have heard that nothing could live in the deepest, darkest, highest-pressure part of the ocean; in fact, snailfish live deeper than any other fish scientists have found, in the deepest trenches. Before 2022, the deepest snailfish seen was a Mariana snailfish (Pseudoliparis swirei) at 8,178 meters in the Mariana Trench in May 2017. 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, and the deepest fish ever captured, two P. belyaevi snailfish, came from 8,022 meters (26,319 feet) in the Japan Trench in August 2022; a 2012 study found that Mariana Trench amphipods have powerful wood-busting enzymes, which let them eat sunken plant debris. The Smithsonian says scientists have found about 25 species of bone-eating Osedax worms since they were first discovered in 2002, and think many more exist, as of November 2026.

In 2012, scientists from Japan filmed a giant squid alive in its deep-sea home for the first time. The 2012 footage of an adult giant squid in the wild was taken near Chichi Island in the North Pacific, from a submersible that followed it to about 900 meters. 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; giant squid can be up to about 13 meters (43 feet) long. Using the size of beaks found in sperm whale stomachs, scientists estimate a giant squid could reach about 66 feet (20 meters), though the longest measured is about 43 feet. A giant squid has eyes as big as dinner plates, and its huge eye, up to 27 centimeters across, is thought to let it spot a sperm whale from more than 120 meters (nearly 400 feet) away.

Mapping is the other half of exploring. As of April 2026, only about 28.7 percent of the ocean floor had been mapped in detail, and Seabed 2030 is a project aiming to produce a complete map of the ocean floor by the end of the decade. NOAA says seabed data are foundational for determining how the ocean works, and mapping underwater volcanoes helps scientists predict tsunamis. NOAA says explorers have seen less than 0.001 percent of the deep ocean seafloor, as of April 2026; the hadal zone holds places so deep that only a handful of humans have ever traveled there. Scientists think most kinds of ocean life have not yet been discovered or named. Estimates of undiscovered ocean species differ, as of November 2026: one NOAA page says 91 percent have yet to be classified, while others say roughly two-thirds or more have not been discovered or described. What else lives at the bottom of the ocean?

Words to know

• hadal: deepest zone
• chemosynthesis: food from chemicals
• bioluminescence: light from animals
• submersible: small submarine
• hydrothermal: hot-water