France, June 4 -- 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 c...