The ocean covers about 70% of Earth’s surface, yet its hidden spaces still outnumber the places people have studied closely. The answer to how much is undiscovered depends on the question being asked. A satellite map, a sonar survey, a camera dive and a biological sample each reveal a different part of the same vast world. The numbers often quoted in this debate describe separate kinds of knowledge rather than competing answers.
NOAA Ocean Exploration says the ocean’s surface area is about 360 million square kilometers and its average depth is 3,682 meters. Modern maps are steadily improving, but mapping alone cannot show every animal and shipwreck. Nor can a map reveal every rocky ledge or relationship within an ecosystem. A patch of bottom may have a measured depth while its geology, living community and past remain unexamined. That difference explains why a large mapped percentage and a tiny visually observed percentage can both be accurate. Much of the ocean remains unknown in the most detailed and direct sense.
Mapping, seeing and sampling are different
“Undiscovered” can sound like a single measurement, but ocean scientists use several measures. A map describes the shape of the seafloor. A camera or human-occupied vehicle can document a place directly. Nets, water bottles and small pieces of tissue can reveal life that a camera misses. A rock sample can reveal an eruption history that neither an image nor a depth measurement provides. Researchers also work at different scales. A global grid can guide regional planning, while a local survey can identify a safe route or a promising dive site. Ocean exploration therefore expands a library of maps, images, specimens and measurements. Each method answers a useful question and none can stand in for all the others.
Maps are especially valuable because they help crews decide where to go. They can reveal ridges, canyons, volcanoes and possible habitats. Researchers can then send remotely operated vehicles to examine a steep slope or a seamount in much finer detail. A detailed map gives explorers a route, while the dive supplies the close look. It also helps a limited ship expedition spend its time at sites where geology or habitat is likely to be especially interesting.
Life adds another layer of uncertainty. A broad slope may appear familiar in a map, yet its living community can still be poorly known. Corals, sponges, worms and microbes may all await study there. The water column, the enormous body of water between the surface and the seafloor, also holds animals that never appear on a bottom map. Seasonal changes can move those animals through the same area at different depths. For that reason, a claim that the whole ocean is explored would need many kinds of evidence.
A global map still lacks fine detail
Every part of the seafloor has a broad global map, largely because satellites can detect subtle changes in sea-surface height caused by gravity from mountains and trenches below. Those measurements are helpful for a planet-wide view. A tall undersea mountain pulls slightly more strongly on nearby water, producing a clue that researchers can turn into a depth estimate. Their resolution is limited, however, so small features can remain blurred or absent. The method is powerful for filling enormous gaps, but it cannot replace a close survey from a ship.
Ship surveys provide the sharper view. A vessel carrying multibeam sonar sends sound pulses toward the bottom and measures their return. The result is a detailed picture of depth, also called bathymetry. It can show the outlines of seamounts and channels that are too small for broad satellite-based maps to resolve. A survey ship must travel back and forth across an area, so covering a remote basin or a rugged coast requires time. Sharing survey data can make each completed voyage useful far beyond its original purpose.
Progress is real and measurable. In April 2026, Seabed 2030 reported that 28.7% of the world’s ocean floor had been mapped to modern standards, or nearly 104 million square kilometers. The project added almost five million square kilometers in the preceding year. In U.S. waters, the NOAA Coast Survey reports about 56% modern mapping coverage. Director Jamie McMichael-Phillips said, “This update reflects what the global community can achieve when data is shared openly and collaboratively.”
The deep seafloor is barely seen
Detailed maps should not be confused with a close-up visit. A 2025 analysis in Science Advances estimated that people have visually observed less than 0.001% of the deep-ocean seafloor. NOAA Ocean Exploration compares that area with roughly the size of Rhode Island. The figure describes direct visual records, which are far harder to gather than satellite data.
Deep dives require specialized ships and pressure-resistant vehicles. They also need cameras, lights and skilled operators. A remotely operated vehicle can travel along a ridge and film animals in place. It moves slowly and covers a narrow path, which gives scientists valuable detail but limits the total area seen during a mission. During a dive, the team must navigate safely and document observations. It must also collect only the samples that can be handled and stored properly. Weather, ship time, distance from port and depth all shape where expeditions can work.
Visual records matter because they show context. A sonar map may reveal a mound. A dive can then show bare rock, a coral garden, a sediment field, or an archaeological site. Images can also document how animals use a habitat. For later researchers, a useful record includes the location and depth. Its date and surrounding conditions matter too. Those details help teams compare one observation with another and return to a site when new questions arise. The Science Advances estimate shows the scale of the gap in visual observations, even as mapping surveys continue to expand.
Most ocean life remains unknown
The unknown is biological as well as geographic. A Current Biology analysis estimated that the ocean may contain roughly 700,000 to 1 million animal species, excluding the far larger microbial world. The study also suggested that about two-thirds of marine animal species may still await formal description. These are estimates, so the actual total can change as records improve.
Describing a species takes more than spotting an unusual creature. Scientists compare features, genetic information and earlier records. They also need specimens or strong evidence that allows other researchers to recognize the organism. A formal description gives future work a stable reference point, so sightings from different expeditions can be compared with care. That careful process helps prevent the same species from receiving several names and it explains why marine species discovery takes time.
New names are only one part of the work ahead. Researchers also want to learn where organisms live and how they feed. They study how warming, low oxygen and pollution may affect them. Fishing and deep-sea disturbance add further questions. A specimen collected once can establish that an organism exists, while repeated observations can begin to show its role in a food web or habitat. Carefully archived maps, video and samples also let future teams revisit a site with better instruments or different questions. The growing record helps scientists separate a one-time observation from a pattern that appears across seasons or regions. These records can inform future expeditions and long-term ocean stewardship. Global maps and camera dives gradually turn blank spaces into places with shape and life. Samples add biological and geological history. The ocean’s remaining unknowns are vast, but each expedition makes the next question more precise.
