Scientists have now made the most comprehensive tally yet of deep-sea exploration — 43,681 dive records dating back to 1958 — and found that humans have directly seen less than 0.001% of the deep ocean floor, an area roughly the size of Rhode Island, leaving the vast seafloor across 66% of Earth’s surface essentially unobserved.

Humanity has sent cameras to the Moon, Mars, Venus and the outer planets, but on Earth itself there is still a larger frontier that has barely been looked at directly. A 2025 study in Science Advances has now put a number on that gap: since 1958, researchers have compiled 43,681 records of dives deep enough to see the seafloor below 200 metres, and all of that visual exploration adds up to less than 0.001% of the deep ocean floor.

That is not a rounding error in the wrong direction. It is an area roughly the size of Rhode Island, or about a tenth of Belgium, set against the part of the planet that lies below the reach of sunlight and covers about 66% of Earth’s surface. In other words, nearly two-thirds of the world is deep ocean, and our direct visual sample of its floor is barely a pinprick.

The paper, led by Katherine L. C. Bell of Ocean Discovery League with Kristen N. Johannes, Brian R. C. Kennedy and Susan E. Poulton, is titled How little we’ve seen: A visual coverage estimate of the deep seafloor. Its central point is simple but uncomfortable: even after the age of submarines, remotely operated vehicles, autonomous vehicles and global oceanographic expeditions, the human view of the deep seafloor remains extraordinarily small.

Seen, not just mapped

The distinction matters. The study is not saying that scientists have no maps of the ocean floor. Satellite gravity data, shipborne sonar and global bathymetry projects have traced the broad architecture of seamounts, trenches, ridges, abyssal plains and continental margins. Those tools tell researchers a great deal about shape and depth.

But visual observation is different. A camera, a crewed submersible or a remotely operated vehicle can reveal whether the seafloor is bare sediment, living coral, hydrothermal vent, sponge field, nodule plain, carcass fall or habitat that has never been described. It can show organisms, behaviour, disturbance, debris and fine-scale structure that do not appear in a coarse map.

Bell’s team focused on that kind of direct in situ visual evidence. They gathered dive records dating back to 1958, the era of early deep submergence vehicles such as the bathyscaphe Trieste and later the research submersible Alvin. The resulting database spans decades of human and robotic exploration, but it still represents only a tiny fraction of the deep seafloor.

A small and biased sample

The number becomes even more striking when geography is added. According to the study, 65% of the visual seafloor observations in the dataset occurred within 200 nautical miles of only three countries: the United States, Japan and New Zealand. Ninety-seven percent of compiled dives were conducted by five countries: the United States, Japan, New Zealand, France and Germany.

That does not mean those countries have done anything wrong. It reflects the cost and difficulty of deep-ocean exploration. Research vessels, pressure-rated vehicles, pilots, engineers, launch systems, cameras, lights, navigation tools and data teams are expensive. Only a small number of institutions have historically had the money and equipment to visit the deep seafloor repeatedly.

The consequence is that the world’s picture of the deep ocean has been shaped by where those vehicles could go, where ships were scheduled, where national programs had priorities and where scientific questions happened to be funded. The abyssal Pacific, Indian Ocean, Southern Ocean and large areas beyond national jurisdictions remain far less visible than their size or ecological importance would suggest.

That matters because sampling bias can quietly become scientific bias. If most visual records come from a handful of regions, researchers may mistake local patterns for global ones. A seafloor that looks sparse in one well-studied area may not represent another. A habitat that seems rare may simply be under-photographed. A species that appears absent may have never had a camera pass nearby.

Why the gap matters now

The timing of the tally is not accidental. The deep ocean is increasingly central to debates over climate, biodiversity, fishing, carbon cycling, biotechnology and deep-sea mining. Some of the areas attracting commercial attention, including nodule-rich plains, are also among the least directly observed ecosystems on Earth.

Visual baselines are the starting point for any serious decision about disturbance. Before researchers can judge whether mining, trawling, cable laying or other activities have damaged a place, they need to know what lived there before the disturbance began. Before regulators can protect a habitat, they need to know where it is. Before scientists can measure recovery, they need images and records from the beginning.

The new estimate makes clear how thin that baseline still is. Even if the total were wrong by an order of magnitude, the authors note, the visually observed area would remain a minute fraction of the deep seafloor. The problem is not that the ocean has been ignored by science. It is that the ocean is so large, so deep and so expensive to visit that decades of work can still leave nearly all of it unseen.

The planet’s largest blind spot

Below 200 metres, sunlight fades quickly. Pressure rises by roughly one atmosphere every 10 metres. At several kilometres down, vehicles must survive crushing force, darkness, cold and a communication problem that space probes do not face in the same way: radio waves do not travel well through seawater, so underwater robots often depend on tethers, acoustic signals or stored instructions.

Those conditions help explain why the numbers are so low. A single dive can require a ship, a trained team and days of planning. Cameras may cover only narrow strips of seafloor. A remotely operated vehicle can spend hours crawling through an area that is enormous to a human observer but nearly invisible on a global map.

The study’s authors argue that this is also an opportunity. Lower-cost vehicles, improved sensors, shared data standards and more distributed exploration programs could widen participation beyond the small group of countries that have dominated the historical record. If coastal states, island nations and local research communities can deploy more accessible tools, the visual map of the deep ocean could become less concentrated and more representative.

For now, the figure is a reminder of scale. The deep seafloor is not a remote corner of the planet. It is most of the planet. Yet the part humans have directly seen is comparable to a small U.S. state, scattered across decades of dives and concentrated near a few technologically wealthy nations.

That may be the most unsettling result of the tally. The largest habitat on Earth is not hidden because it is far away. It is hidden because it is under us, in the dark, and we have only just begun to look.

Sources

Bell et al., How little we’ve seen: A visual coverage estimate of the deep seafloor, Science Advances, 2025
Science Advances article page
TIME report on the Science Advances study
NOAA Ocean Exploration data access
National Deep Submergence Facility dive and vehicle data