Only a tiny % of the deep seafloor has ever been visually observed: Study

For national and international policymakers, decisions on how to regulate — and whether to allow — exploitative human activities such as deep-sea mining require good information. A new study suggests that when it comes to the ocean floor, they may not have it.

Only 0.001% of the deep seafloor has ever been captured by photo or video images, the study found. And that which has been captured is “biased” and potentially unrepresentative: 65% of observations have been in the waters of the United States, Japan or New Zealand, according to the study, which was published in the journal Science Advances on May 7.

Lead author Katy Croff Bell, president of Ocean Discovery League, a U.S.-based nonprofit, said the study shows how much is still unknown about the deep sea and how much more research is required.

“We’ve only seen 0.001%, and look at how much we know [from that] 0.001%,” she told Mongabay. “How much do we not know that’s yet to be discovered?”

The deep sea is one of the hardest areas on Earth to study. The crushing pressure created by gravity, along with absolute darkness, make it hard to reach and navigate safely.

“We know more about the dark side of the moon than we do about this, the deep seafloor,” Kenneth Macdonald, a marine geophysicist and professor emeritus at University of California Santa Barbara who was not involved in the study, told Mongabay.

Researchers have used seafloor mapping, which involves the use of sonar, more than imaging, largely because the former is more practical: Sound waves travel farther than light waves. More than a quarter of the seafloor has been mapped, thanks largely to a major international project called Seabed 2023 that is currently underway.

Seafloor imaging, which began in 1958, is a slower undertaking: Many of the remote operating vehicles (ROVs) that are used to capture imagery move at below 1 mile per hour and have a visual field just 10 meters (33 feet) wide. Not surprisingly, it’s also very expensive. The ROVs cost millions of dollars each and often require big research vessels to bring them out to sea, where it can then cost roughly $100,000 a day to run seafloor operations, Bell said.

Given the logistical challenges, it’s no surprise that seafloor imaging has been limited. But until now, just how limited hasn’t been known. Data on observations were spread out at institutions all over the world, and no one had ever collected and analyzed them globally.

“Nobody had ever done the math before,” Bell said.

Bell and her team didn’t collect the photos and videos of the seafloor — building such a central repository or archive would be a separate project — but rather the data on those observations: where they were taken, how many dives and types of vehicles used.

From that database, they used two complementary methods to determine the percentage of the deep seafloor that had been imaged. One was a dive-based method: They estimated the size of area covered per dive for each type of ROV and extrapolated. They found that a maximum of 2,130 square kilometers (822 square miles) had been covered — well under 0.001% of the deep seafloor.

The authors’ more conservative 0.001% estimate in fact comes from the second method, which was based on the total time in the water and at the seafloor (“bottom time”), taking into account ROVs’ speed and the width of their visual fields. Using this method, they calculated that total seafloor coverage since 1958 has been 3,823 square kilometers (1,476 square miles). That’s about 0.001% of the deep seafloor, which covers two-thirds of the Earth’s surface. (They define the deep seafloor as that found more than 200 m, or 656 ft, beneath the surface.)

Bell said there are surely seafloor observations that didn’t make into the database, but she and her team made efforts to counter that by overestimating the visual coverage figures. In any case, even if their finding was off by an order of magnitude, it would still be powerful, she said.

Larry Mayer, a geophysicist at the University of New Hampshire who is one of the leaders of the Seafloor 2023 mapping project, and who wasn’t involved with the new study, told Mongabay that it was a “reminder to people about how big the ocean is and how little we know about it.”

Mayer said that imaging is a way to “groundtruth” other techniques: Photo and video offer fine-grained details that mapping can’t.

Matt Gianni, co-founder of the Deep Sea Conservation Coalition, an umbrella group of NGOs, agreed with Mayer about the importance of visual observation, saying that it offers key data on “life history characteristics, community structures and the connectivity of both species and ecosystems.”

Gianni emphasized that it’s important for scientists to explain to regulators and policymakers that which isn’t known. In such cases, decision-makers are sometimes legally bound to take a more precautionary approach when assessing the impacts of human activities, he said. The U.N. Fish Stocks Agreement, for example, requires member states to “be more cautious when information is uncertain, unreliable or inadequate. The absence of adequate scientific information shall not be used as a reason for postponing or failing to take conservation and management measures.”

Lisa Levin, a biological oceanographer and marine ecologist emeritus with the Scripps Institution of Oceanography at the University of California San Diego, told Mongabay that a wide range of international institutions should bear the new study’s findings in mind, including the regional fisheries management organizations that manage deep-sea fishing, the parties to the London Convention and Protocol that govern, among other activities, the potential use of marine carbon dioxide sequestration systems, and the parties to the Convention on Biological Diversity, which is implementing a “30×30” initiative that aims to safeguard 30% of the planet’s land and sea with official protections by 2030.

The study comes in the middle of international debate over deep-sea mining, which is governed by the International Seabed Authority (ISA), though U.S. President Donald Trump recently moved to circumvent that body and issue deep-sea mining permits unilaterally. (A spokesperson for the U.S. State Department told Mongabay that “President Trump is prioritizing the revitalization of domestic maritime industries … and establishing the United States as a global leader in responsible seabed mineral exploration and processing.”) Several experts told Mongabay that the ISA and U.S. authorities need to consider the lack of research that’s been done before issuing permits. Likewise, Bell said Ocean Discovery League’s position on activities like deep-sea mining is that more research needs to be done first; she said the group’s focus is research, not advocacy, however.

Bell said the second major takeaway from their research was the high level of geographic bias in the results. Certain dive sites have gotten a disproportionate amount of research attention. Monterey Canyon on the U.S. West Coast was surveyed 3,530 times, nearly as much as all other canyons in the world combined. The U.S., Japan and New Zealand not only were home to most of the deep-sea dives but also, along with France and Germany, led the vast majority of the research: 97.2%. And the remaining 2.8% was also mostly done by high-income countries.

Deep-sea research dominance by high-income countries is slowly changing: A “diversity of operators” is starting to emerge, the study says. That could lead to a wider range of areas and geological features surveyed.

“Part of that is because of right now we’re seeing [an increase in] lower cost, more accessible technologies to be able to study the deep sea, so that’s going to hopefully decrease some of the biases that we see and enable more people in more places to explore,” Bell said.

Citation:

Bell, K. L., Johannes, K. N., Kennedy, B. R., & Poulton, S. E. (2025). How little we’ve seen: A visual coverage estimate of the deep seafloor. Science Advances, 11(19). doi:10.1126/sciadv.adp8602

This article by Edward Carver was first published by Mongabay.com on 30 May 2025. Lead Image: Two lithodid crabs exhibiting mating behavior while perched on top of a basaltic rocky outcrop with an anemone (Liponema sp.) and a white sponge in the background. This photograph was taken by a remotely operated vehicle in the Gulf of Alaska in 2023 as part of a NOAA expedition. Image courtesy of NOAA. 

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