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Water covers more than 70% of Earth’s surface, yet NOAA reports that as of April 2026 only about 28.7% of the global seafloor had been mapped with modern high-resolution sonar. Less than 0.001% of the deep-ocean seafloor has been directly seen by explorers, an area comparable to Rhode Island.
Over 90% of the ocean is deeper than 200 meters, so most of it sits beyond the reach of sunlight. Here are five facts that show how much remains to be learned, plus two closer looks at the dark and the depths.
1. We’ve mapped more of the Moon and Mars than the ocean floor
NOAA’s current estimate is that roughly 28.7% of the global seafloor has been mapped with modern multibeam sonar, with about 56% of U.S. waters covered. Satellites cover the entire seafloor, but only in general terms. A figure like this depends on what “explored” means: mapping with sonar and actually seeing the bottom with a camera or a submersible are very different levels of knowledge.
The Moon and Mars comparison holds for detail, not for coverage. In a widely cited comparison, the whole Moon and Mars surfaces were mapped at about 100-meter resolution, while the global seafloor map could resolve features only about 5 kilometers wide. Mapping a planet from orbit is easier when nothing is in the way, and the water above the seafloor is a very large obstacle.
2. The “twilight zone” is nearly pitch black
The twilight zone, or dysphotic zone, runs from about 200 to 1,000 meters. Sunlight can be detected down to roughly 1,000 meters under the right conditions, but significant light rarely reaches past 200, and photosynthesis stops being possible. Below 1,000 meters is the aphotic zone, where no sunlight arrives at all.
Many animals here make their own light. A study by MBARI researchers Séverine Martini and Steve Haddock, published in Scientific Reports in 2017, found that about three quarters of the animals in Monterey Bay waters from the surface to 4,000 meters can produce light. That figure comes from one location off California, not a worldwide count, but it shows how common bioluminescence is in the deep.
3. The ocean produces much of the oxygen we breathe
NOAA says the ocean produces about half of Earth’s oxygen, with marine life consuming roughly the same amount. Most of that production comes from plankton, drifting plants, algae and some bacteria that photosynthesize. One tiny bacterium, Prochlorococcus, is credited with up to 20% of the oxygen in the biosphere, more than all tropical rainforests on land combined.
Two caveats keep the headline in perspective. Exact figures are hard to calculate because production shifts with the seasons, nutrients and water temperature. And most of the oxygen people breathe comes from the atmosphere, which built up over hundreds of millions of years, not directly from the ocean’s output today. When algal blooms die, decomposition can use up oxygen faster than it is replaced, leaving low-oxygen “dead zones.”
4. There are mountains and canyons bigger than any on land — just underwater
The mid-ocean ridge stretches about 40,389 miles, or roughly 65,000 kilometers, around the planet, and about 90% of it lies underwater. It is described as the longest mountain range on Earth and the largest single volcanic feature on the planet. Most of the world’s volcanoes lie along its boundaries; see these volcano facts for more.
It forms where tectonic plates pull apart. Magma rises to fill the gap, building mountains and valleys on the seafloor and creating new ocean crust as the surface spreads. That makes the ridge less like a single wall and more like a long seam where the planet’s outer shell is constantly being renewed.
5. New marine species are discovered all the time
Scientists estimate between 700,000 and 1 million ocean species, and roughly two thirds of them have not yet been discovered or formally described. About 2,000 new species are accepted each year, so new discoveries are routine rather than rare.
Considering how little of the deep seafloor has been seen directly, that pace is unsurprising. Familiar groups such as jellyfish and octopuses are well known, but much of what remains undescribed probably lives where few instruments have reached.
Why satellites can map Venus but not the seafloor
Seawater blocks radar and radio waves, so satellites cannot image the seafloor directly the way they map land or other planets. NASA’s Magellan mission mapped about 98% of Venus at roughly 100 meters. For Earth’s ocean, satellites instead measure the height of the sea surface very precisely.
After the effects of waves and tides are removed, the remaining bumps and dips reflect the seafloor below: an underwater mountain adds mass that pulls water into a slight rise, and a trench produces a dip. The global 5-kilometer map was built this way from Cryosat-2 and Jason-1 data. Finer detail needs ships with sonar, which survey narrow strips at about 100 meters.
From the sunlit 200 meters down to the hadal trenches
The ocean is usually divided into layers by depth, and each one gets darker and less visited than the last:
- 0 to 200 meters: the sunlit euphotic zone, home to most commercial fisheries.
- 200 to 1,000 meters: the twilight zone.
- 1,000 to 4,000 meters: the bathypelagic, or “midnight” zone.
- 4,000 to 6,000 meters: the abyssopelagic, or “abyss.”
- 6,000 meters and deeper: the hadopelagic, or “hadal” zone.
This topic also connects with facts about birds and penguins, both worth a read.
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At about 2,000 newly accepted species a year and less than 0.001% of the deep seafloor seen directly, the ocean’s catalog is still being written one dive at a time.