We have mapped the surface of Mars, the Moon and Venus in sharper detail than the floor of our own ocean, most of which no human instrument has ever seen up close

Here is a claim that sounds like an exaggeration and is not: the best global maps of Mars, the Moon and Venus resolve finer detail than the best global map of the floor of our own ocean. The other worlds have been surveyed from orbit down to a few hundred metres, and in places a few metres. Most of the seabed has never been measured directly at anything close to that.

The comparison gets repeated so often that it has started to sound like a slogan. The underlying figures hold up, but they need a little care, because the word “mapped” is doing different work on land and at sea.

What we actually have a map of

There is, in fact, a complete map of the entire ocean floor. It just is not the kind of map most people picture. Almost all of the global seafloor image comes from satellite altimetry, which measures tiny bumps and dips in the height of the sea surface. A seamount has slightly more mass than the water it displaces, so its gravity pulls the surface into a low mound above it. From those surface heights, scientists infer the shape of the rock below.

That method reaches everywhere, but it is coarse. In December 2024, Yao Yu and David Sandwell of the Scripps Institution of Oceanography, with Gerald Dibarboure of the French space agency, reported in the journal Science that data from the NASA and CNES SWOT satellite had produced a gravity map of the seafloor at about eight kilometres resolution. That was roughly twice as sharp as the previous standard built up over thirty years of older satellites. It was enough to reveal thousands of small, previously uncharted seamounts.

Eight kilometres is a genuine improvement. It is also a reminder of the scale. The finest global picture of the seabed resolves features about the size of a small city. Anything smaller has to be inferred, or measured some other way.

How the other worlds compare

Now hold that eight kilometres against the planetary maps.

Venus is wrapped in cloud that no camera can see through, yet NASA’s Magellan spacecraft used radar to map about 98 per cent of its surface between 1990 and 1994, at a resolution of roughly 100 to 300 metres. Mars has been measured in still finer detail. The Mars Orbiter Laser Altimeter on Mars Global Surveyor gathered more than 600 million elevation points between 1999 and 2001, giving a global topographic model at 463 metres per pixel.

The Moon is sharper again. The Lunar Orbiter Laser Altimeter aboard NASA’s Lunar Reconnaissance Orbiter has collected billions of height measurements and produced a global model at 118 metres per pixel, with vertical precision measured in centimetres and much finer coverage near the poles.

So the pattern is real. The complete map of another planet’s surface tends to sit in the hundreds of metres. The complete map of our own seabed sits in the kilometres. On that measure, we do know the face of three other worlds better than the bottom of the sea.

Why water is the problem

The reason is not neglect. It is physics.

Radar and laser altimeters map Mars, Venus and the Moon because their signals travel through vacuum or thin air and bounce cleanly off rock. Seawater stops them. Radar from orbit cannot see the seabed through several kilometres of ocean, which is why the satellite method has to work indirectly, reading the water’s surface rather than the floor.

To measure the seabed directly, you have to go there, or send sound. Ships drag multibeam sonar back and forth, mapping a strip of seafloor beneath them as they go. The results are detailed, often better than 100 metres, but a single vessel covers a narrow lane at a time, and the ocean is large and deep and mostly far from anywhere.

How much has been seen up close

This is where the honest version of the claim matters. The whole seabed has a coarse map. Only part of it has been measured directly at high resolution.

The Seabed 2030 project, a collaboration between the Nippon Foundation and the international GEBCO bathymetric charting effort, tracks this directly. On World Hydrography Day in June 2025 it announced that 27.3 per cent of the ocean floor had been mapped to modern standards, drawing on data from more than 185 organisations. That figure was about 6 per cent in 2017, so the gap is closing quickly, but it still leaves close to three quarters of the seabed unmeasured in the direct sense.

That is the part worth holding on to. It is not that the ocean floor is a blank. It is that for most of it, what we have is an inference from the sea surface far above, not a reading taken up close.

What is changing

Two efforts are narrowing the distance from opposite directions. From orbit, SWOT continues to sharpen the global gravity picture, and its mission is ongoing. On the water, Seabed 2030 aims to assemble a complete high-resolution map of the ocean floor by the end of the decade, stitched together from ships, autonomous vessels and archived surveys.

Whether the 2030 target is met will depend on how much of the remote deep ocean gets surveyed in time, and that is the number to watch. The surface of Venus took one radar mission a few years to map. The floor of our own ocean is turning out to be the harder assignment.