The ocean covers roughly 70 percent of the planet, yet about 80 percent of it has never been directly observed. More of the surface of Mars has been mapped than the floor beneath the waves. The deepest point in the ocean, the Mariana Trench, sits nearly 36,000 feet below the surface. In all of human history, three people have reached its bottom: two in 1960 and one in 2012.

For comparison, more than 600 people have flown into space, and thousands have stood on the summit of Mount Everest. The contrast is often explained by the conditions. At the bottom of the trench, the water pressure is roughly equivalent to 50 fully loaded jumbo jets stacked on a person’s chest. A human body would not survive exposure at that depth; the lungs would collapse and the body would be crushed in seconds. The deep ocean is dense, cold, and unforgiving, and every foot of descent increases the force pushing against anything that enters it.

The descent itself follows a predictable pattern. Sunlight fades gradually until it disappears entirely, leaving permanent darkness below roughly 200 meters. Temperatures drop to just above freezing. In the twilight zone, many animals produce their own light through bioluminescence. Some use it to attract prey, dangling glowing lures in front of their mouths. In that environment, light functions primarily as a hunting tool rather than a source of illumination.

Deeper still, in the midnight zone, many creatures have lost functional eyes altogether. Evolution discarded the organ because there is nothing to see. The food web there depends on marine snow: a slow, continuous drift of dead organic material falling from the sunlit surface far above. This material sustains communities of fish and invertebrates that live in total darkness.

At the bottom of the trench, explorers found complex animals rather than only microbial life. Snailfish, pale and gelatinous, drift near the seafloor at depths of several miles. Large isopods, armored and segmented, move across the sediment. Around hydrothermal vents, where superheated mineral-rich water rises from the Earth’s crust, entire ecosystems have developed. These communities do not rely on sunlight. Bacteria convert chemicals from the vent water into energy, and larger animals feed on them. Scientists have noted that if life exists beneath the icy crust of moons like Europa or Enceladus, it could resemble these vent communities.

The ocean has also produced unexplained events that remain unresolved. In the 1870s, the merchant ship Mary Celeste was found adrift in the Atlantic with its sails set, cargo intact, and personal belongings undisturbed. The crew was gone. One lifeboat was missing. No bodies or survivors were ever found.

A larger case involves the SS Waratah, a passenger steamer that disappeared in 1909 off the coast of southern Africa with 211 people aboard. No distress call was received. Searches found no wreckage, no lifeboats, and no bodies. The ship’s final resting place remains unknown.

In recent years, researchers documented a pattern of great white sharks washing ashore with their livers removed. The injuries were precise, and the rest of the body was left intact. The cause was identified as a pair of killer whales that had developed a technique of flipping the sharks upside down, which induces a state of temporary paralysis, then extracting the liver. Following these incidents, local shark populations vacated the area for an extended period.

Underwater acoustic monitoring has also recorded sounds that have not been fully explained. One recurring sound, detected seasonally since the early 1990s, rises and falls in pitch on a regular schedule. Its source has not been identified. In 2016, hunters in the Canadian Arctic reported a steady pulsing sound coming from the seafloor in a remote channel. Wildlife in the area reportedly left. Military aircraft searched the region and found no identifiable source.

The seafloor itself contains features that remain largely unseen. Between Greenland and Iceland, a massive underwater waterfall carries dense cold water over a cliff and down more than 11,000 feet, the largest known waterfall on Earth. In other locations, pools of hypersaline water sit on the ocean floor, dense enough that they do not mix with the surrounding seawater. The chemistry of these brine pools is lethal to most marine life.

Off the coast of southern Japan, divers found a large stone formation at the Yonaguni site with flat terraces and what appear to be steps. Some researchers suggest it could be a man-made structure submerged when sea levels rose at the end of the last ice age. Others argue the formation is natural, the result of sandstone fracturing along clean lines. The debate has not been resolved.

Estimates suggest the ocean contains between 200,000 and 2 million species yet to be described. As of 2024, roughly a quarter of the seafloor has been mapped in detail. When a camera reached the bottom of the Mariana Trench, it found a plastic bag. Even the most remote depths of the ocean have been affected by human activity.

Most of the deep ocean remains unseen and unexplored. It is a cold, dark, high-pressure environment that is far more difficult to reach than space, and much of what exists there has never been observed.