It is the largest habitat on Earth – and also the least explored. As world leaders prepare to meet in Nice for a major UN summit on the ocean's future, scientists say we still know remarkably little about what lies beneath the waves.
Just 26.1 percent of the global seafloor – including both shallow and deep areas – has been mapped using modern sonar, according to the Seabed 2030 project, which aims to chart the entire ocean floor by the end of the decade.
But mapping from above is not the same as seeing it up close. Scientists estimate that humans have directly observed less than 0.001 percent of the deep seafloor – defined as depths below 200 metres. That’s an area roughly one-tenth the size of Belgium.
That figure comes from a study published this month in Science Advances led by explorer and scientist Katy Croff Bell who, along with colleagues, compiled data from more than 43,000 deep-sea dives carried out since the 1950s.
The results show how lopsided ocean exploration has become. Nearly two-thirds of all observations happened within 200 nautical miles of just three countries: the United States, Japan and New Zealand. Five nations conducted 97 percent of all dives.
This leaves entire regions of the ocean floor completely undocumented – particularly in waters around poorer countries that lack the tools and funding for deep-sea research.
"As we face accelerated threats to the deep ocean – from climate change to potential mining and resource exploitation – this limited exploration of such a vast region becomes a critical problem for both science and policy," Bell, founder of the non-profit Ocean Discovery League, told Scientific American.
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Charting the unknown
Some of those gaps are starting to close thanks to new tools.
NASA’s SWOT satellite – short for Surface Water and Ocean Topography – was launched in December 2022 to track changes in water height across oceans, rivers and lakes.
By measuring tiny shifts in sea surface elevation – sometimes just a few centimetres – it helps scientists detect what lies below, including underwater mountains, ridges and deep-sea trenches.
A study published in the journal Science last December found that SWOT delivered clearer images of the seafloor in a single year than earlier satellites achieved in three decades.
“In this gravity map made from merely one year of SWOT data, we can see individual abyssal hills, along with thousands of small uncharted seamounts and previously hidden tectonic structures buried underneath sediments and ice,” said Yao Yu, a postdoctoral researcher at Scripps Institution of Oceanography.
“This map will help us to answer some fundamental questions in tectonics and deep ocean mixing.”
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Why mapping matters
But maps like these do more than fill scientific gaps. They help pinpoint safe sites for offshore wind farms, guide where to lay submarine cables and flag areas at risk from tsunamis or underwater landslides.
These kinds of insights are becoming central to marine policy – especially as countries look to balance economic development with protecting the ocean.
Still, many scientists say there’s no substitute for a direct look. Visual dives don’t just show topography – they reveal entire ecosystems, offering clues about what species live there, how they interact and how fragile they may be.
“Being able to explore, or at least accelerate, the exploration of the other 99.999 percent of the deep ocean is really going to give us an amazing opportunity to ask new questions we’d never even thought of before,” said Bell.
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Eyes on the deep
New expeditions are already pushing into the deep.
This year, the research vessel Nautilus, operated by the Ocean Exploration Trust, is exploring the Mariana Islands – a region dotted with more than 60 underwater volcanoes.
Scientists are using remotely operated vehicles to study hydrothermal vents and collect biological and geological samples from depths of up to 6,000 metres.
Further north, teams led by the US National Oceanic and Atmospheric Administration (NOAA) are surveying the Aleutian Arc off Alaska, where only 38 percent of the seafloor has been mapped. They’re studying deep-sea coral habitats, volcanic formations and possible mineral deposits.
These missions are part of a growing global effort to unlock the secrets of the deep – an environment that helps regulate climate, store carbon and sustain biodiversity.
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High-stakes summit
The ocean feeds 3.2 billion people and generates an estimated $2.6 trillion in economic value each year. Yet just 8 percent is formally protected – and only a fraction of that is off-limits to damaging activities.
That disconnect will be centre stage in Nice, where world leaders, scientists and campaigners are meeting for the third UN Ocean Conference (UNOC) from 9 to 13 June.
Co-hosted by France and Costa Rica, the summit follows a string of high-level events already under way.
More than 2,000 scientists are taking part in the One Ocean Science Congress this week, while the Blue Economy and Finance Forum in Monaco this weekend will bring together investors and policymakers to address the multi-billion-dollar funding gap in marine protection.
A public exhibition area called La Baleine has been open since Monday at Nice’s Palais des Expositions, while the Ocean Rise and Coastal Resilience Coalition summit on Saturday will focus on coastal communities affected by rising seas.
The goal in Nice is to secure new voluntary commitments under the Nice Ocean Action Plan – pledges from governments, businesses and civil society to protect marine life and support the sustainable use of the seas.
But for many researchers, it starts with something more basic: actually knowing what’s down there.
