Science · 4 min read

We have better maps of Mars than of our own seafloor

Most of the ocean floor has never been directly measured. What we have is inferred from the shape of the sea surface.

The familiar image of the seafloor — ridges, trenches, abyssal plains in blue relief — looks like a map. For roughly three quarters of the ocean it is closer to an educated reconstruction, and its resolution is coarse enough to hide a mountain.

Why the ocean is hard

Light does not travel far in water and neither do radio waves, which rules out the satellite imaging that mapped the land and the surface of Mars. Direct measurement means sound: a ship, a multibeam sonar, and a slow pass back and forth. A modern survey vessel covers a corridor a few kilometres wide at roughly the speed of a bicycle.

The ocean covers about 360 million square kilometres. Surveying that at ship speed is a multi-decade, multi-billion-dollar undertaking, which is why after a century of effort a substantial fraction has still never had a ship pass over it with sonar running.

The trick with the sea surface

The global maps exist because of an elegant substitution. A seamount is a large mass, and its gravity pulls water toward it, raising the sea surface above it by a few centimetres to a metre. A trench does the reverse. Satellite altimeters can measure sea surface height precisely enough to detect these bumps.

Invert the relationship and you can infer the shape of the bottom from the shape of the top. This is how most of the seafloor was mapped, and it works remarkably well at large scales. Its limit is resolution: the technique resolves features around a kilometre or two across at best. Anything smaller is invisible.

What hides in the gap

Plenty. In 2005 the submarine USS San Francisco struck an undersea mountain at full speed that did not appear on its charts. Ships still occasionally find seamounts rising hundreds of metres that no map showed, simply because nothing had ever passed directly over them.

The gaps matter beyond navigation. Seafloor shape governs how deep water circulates, which governs how the ocean moves heat around the planet — a first-order input to climate models. Tsunami prediction depends on bathymetry too, since the shape of the bottom determines how a wave grows as it approaches shore.

The effort to close it

Seabed 2030 is an international project aiming for a complete high-resolution map by the end of the decade. Progress has been faster than expected, largely by pooling data that already existed in fragments — commercial surveys, naval archives, fishing fleets, research cruises — rather than by surveying everything fresh.

Coverage has moved from around six percent at the start to roughly a quarter. The remainder is mostly deep open ocean, far from any route a ship has reason to take, which is precisely why it stayed blank in the first place.