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 the hadal zone holds places so deep that only a handful of humans have ever traveled there. Long before anyone could travel to the bottom, people were trying to measure how far down it was. How, then, does anyone know what lives at the bottom of the ocean? The answer comes from tools and measurements made by ships, explorers and machines, and a sounding is a measurement of how deep the water is. The first attempt to sound the Mariana Trench came during the Challenger Expedition (1872 to 1876); in 1875 a sounding of 8,184 meters was made near its southern end.

The Mariana Trench is a long, deep trench in the floor of the western Pacific Ocean, near Guam, and it is an arc-shaped trench more than 2,540 kilometers (1,580 miles) long, with an average width of about 69 kilometers (43 miles). The Mariana Trench formed by subduction, where the Pacific Plate is forced below the smaller Mariana Plate; the deepest place in the ocean is called Challenger Deep, and NOAA gives its depth as about 10,935 meters (35,876 feet), as of November 2026. The average ocean is about 3,682 meters (12,080 feet) deep, as of November 2026; the hadal zone is the deepest part of the ocean, found in deep trenches below about 6,000 meters (19,700 feet).

Depth gives the ocean its layers; 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. 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. 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. Deep-ocean life is under extreme pressure, from about 40 to over 110 times the pressure of Earth's atmosphere. Little is known about the animals in the midnight zone, and even less about its microbial life. 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.

How do people actually get to the bottom, and who has been there? Two kinds of vehicle do the work: those that carry people and those that are steered from a ship. 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, 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. NOAA Ship Okeanos Explorer carries a two-part remotely operated vehicle that can dive to 6,000 meters, as of November 2026, and the ship is described as the only federal vessel dedicated to exploring our largely unknown ocean.

What happened after Trieste, and how did later dives compare? In 2012, the filmmaker James Cameron dived alone in a one-person submarine, Deepsea Challenger, to the bottom of Challenger Deep. Deepsea Challenger is a one-person submersible, which is a small submarine built to carry people underwater, 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, where pressure sensors averaged 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.

What has been filmed down there? 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; a giant squid's 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. Giant squid can be up to about 13 meters (43 feet) long; 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.

How do scientists count animals in the dark? Video has been one way; an MBARI study of video from 240 dives found that about three quarters of the animals in Monterey Bay waters from the surface to 4,000 meters can 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, 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 that lure holds bacteria that make light. 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.

Instruments left on the sea floor measure what falls to it. Sediment traps on the ocean floor show that about 815 million tons of carbon reach the ocean floor each year, mostly as marine snow. 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.

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. 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. 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. 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.

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. 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. Osedax worms house bacteria in their root-like parts that use sulfur in whale bones to make energy by chemosynthesis. Up to 190 kinds of bacteria have been found on one whale carcass; some whale falls can support a blanket of 45,000 worms per square meter, the highest animal density in the entire ocean.

Which fish live deepest, and how deep is that, as far as anyone has found? Fish records show how far the search has reached. 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. 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.

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. 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 also says we know less about the ocean floor than we do about the surface of the Moon and Mars, and that explorers have seen less than 0.001 percent of the deep ocean seafloor, as of April 2026. 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 will the next map and the next dive turn up?

Words to know

• sounding: depth measurement
• submersible: small submarine
• hadal: deepest zone
• chemosynthesis: food from chemicals
• sediment: settled material