We've Mapped Other Planets. What About Our Own Ocean Floor?

We have a global topographic map of Mars at a resolution of a few hundred metres, and selected areas imaged at better than a metre. The Moon is mapped globally to similar precision. Venus, whose surface is permanently hidden beneath cloud, was radar-mapped by the Magellan spacecraft in the early 1990s at roughly 100 to 200 metres.

As of the 2026 release of the GEBCO global grid, 28.7 percent of Earth's ocean floor has been mapped to modern standards. The remaining 71.3 percent, covering roughly half the surface of the planet, exists on our charts as interpolation and inference.

This is not a failure of ambition or funding, or not only that. It is a consequence of physics, and understanding why reveals something useful about why the gap has been so stubborn and what is now closing it.

 

Why the Seabed Is Harder Than Mars

 

Everything we know about the surface of other planets, we learned by bouncing electromagnetic radiation off them: visible light, infrared, radar. From orbit, a single instrument can image an entire world in a few years.

Seawater does not permit this. Visible light is absorbed within the first tens of metres and gone entirely by about 200. Radar, which penetrated Venus's clouds without difficulty, is absorbed by water almost immediately. There is no wavelength that will travel four kilometres down through seawater, reflect off rock and come back carrying useful information. The ocean is opaque to the entire method by which we mapped the solar system.

So the seabed cannot be surveyed from a distance. It has to be surveyed from close range, by something physically present above it, moving at the speed of a ship. That single constraint explains the whole shape of the problem: it is not that nobody tried, it is that the only direct technique available requires putting a vessel over every square kilometre of a 361 million square kilometre ocean.

 

What 28.7 Percent Actually Means

 

The figure comes from the Nippon Foundation and GEBCO Seabed 2030 project, which announced in 2026 that global coverage had reached 28.7 percent, equivalent to about 104 million square kilometres, an area the project notes exceeds two thirds of Earth's land surface.

The phrase "to modern standards" does real work and is worth unpacking. Essentially all of the ocean appears on charts, because gaps are filled by interpolation and by low-resolution estimates. What is missing is direct measurement at the grid resolution Seabed 2030 defines, which varies with depth, from 100 metres in shallow water to 800 metres in the deepest basins. A chart showing a featureless abyssal plain may be showing genuine flatness, or may be showing that nobody has ever measured there.

The progress is genuine and accelerating. Coverage stood at around 6 percent when the project began in 2017 and roughly 25 percent in 2024. The 2026 grid added almost 5 million square kilometres in a single year, a record. Contributions now come from 220 organisations, 15 of them joining in the past year, with Malaysia, Morocco, Papua New Guinea and Saudi Arabia among first-time contributors. Some regional gains have been dramatic: coverage of the ROPME Sea Area rose from about 6.4 percent to 20.5 percent in one year.

One nuance that rarely gets mentioned. A substantial share of each year's addition comes not from new surveying but from existing data being released into the public grid by navies, hydrographic offices, energy companies and research institutions that had collected it for their own purposes and never shared it. That is enormously valuable and also finite. The archives will eventually empty, and what remains will have to be surveyed from scratch.

 

Scientists Use Sound to Reveal the Seabed

 

Where light fails, sound works. Water carries acoustic energy efficiently, which is why whales can communicate across ocean basins and why sonar exists.

A multibeam echosounder mounted on a ship's hull emits a fan of acoustic pulses perpendicular to the direction of travel and measures the time each takes to return. Multiply by the speed of sound in seawater, correct for how that speed varies with temperature, salinity and pressure through the water column, and you have depth at hundreds of points across a swath beneath the vessel. Sail forward and the swaths build into a continuous three-dimensional surface. Modern systems resolve features at roughly 200 to 400 metres in deep water, which is fine enough to see individual seamounts, canyons, fault scarps and landslide scars.

The limitation is throughput. Swath width scales with depth, typically around three to four times the water depth, so a ship in 4,000 metres of water images a strip maybe 12 to 16 kilometres wide while moving at perhaps 10 knots. Even running continuously, a single vessel covers a few hundred thousand square kilometres in a year at best. A research ship costs tens of thousands of dollars a day to operate. The arithmetic of mapping an ocean one ship at a time is brutal, which is precisely the problem the other two technologies in the carousel's fourth slide are there to address.

 

Satellites Reveal Shapes, Robots Capture Details

 

These two approaches are often mentioned in the same breath as if they were alternative solutions to the same problem. They are not, and the distinction is the most useful thing to understand about modern seabed mapping.

Satellite altimetry does not see the seabed at all. It measures the height of the sea surface, extremely precisely. A seamount is a large mass of rock, and its gravitational attraction pulls water toward it, raising the sea surface above it by a few tens of centimetres in a broad mound. Measure those bumps and you can infer the mass distribution below, and from that estimate the topography. It is indirect, it works everywhere the satellite passes, and it is fast.

It is also coarse. Marine gravity maps built from three decades of satellite data resolve features at roughly 12 to 16 kilometres. The SWOT mission, which measures sea surface height in two dimensions rather than along a single track, has improved this to about 8 kilometres and in doing so revealed thousands of previously unknown small seamounts. That is a genuine scientific advance. But 8 kilometres against the 200 to 400 metres a ship achieves is a difference of twenty to forty times in linear resolution, and hundreds of times in area. Satellite altimetry finds features. It cannot chart them, and it will never satisfy the Seabed 2030 standard. It is reconnaissance, telling survey vessels where to look.

Uncrewed surface vessels attack the cost problem instead. A small autonomous vessel carrying a multibeam system collects data of comparable quality to a crewed ship, with no crew to accommodate, feed or rotate, far lower fuel consumption, and the ability to stay on station for weeks. The resolution is the same because the physics is the same. What changes is the cost per square kilometre and the number of platforms that can be in the water simultaneously. Fleets of these vessels are now being deployed on national mapping programmes, and they are the main reason the annual addition rate is rising.

So the division of labour is: satellites say where the interesting things are, robots and ships go and measure them properly. Neither replaces the other.

 

The Arithmetic of 2030

 

Here is a calculation worth doing out loud, because it clarifies what the target actually requires.

The ocean covers roughly 361 million square kilometres. At 28.7 percent coverage, about 257 million square kilometres remain unmapped. The record year just achieved added almost 5 million. Sustained at that record pace, completing the map would take somewhere in the region of fifty years, putting the finish line nearer 2077 than 2030.

The project's own framing is not that 2030 marks completion of surveying but that it marks the deadline for assembling a complete map by every available means, including modelled and interpolated data where direct measurement does not exist. UNESCO's Intergovernmental Oceanographic Commission has separately pledged to have at least 80 percent of the seabed mapped by 2030, and has costed that at roughly 5 billion dollars, or about 625 million a year, requiring a dedicated fleet of around 50 vessels, far wider use of autonomous sonar, and governments and companies releasing cartographic data they already hold.

That last item is the cheapest line in the budget and possibly the most consequential. A significant quantity of high-quality bathymetry already exists inside naval archives, offshore energy datasets and commercial survey holdings, withheld for reasons of security, competitive advantage or simple inertia. Unlocking it costs almost nothing compared with sending a ship.

None of this makes the project unserious. Going from 6 percent to 28.7 percent in under a decade, while building the international data pipeline to do it, is a substantial achievement. But the gap between the rhetoric of a finished map by 2030 and the arithmetic of ship time is wide, and the honest version is more persuasive than the optimistic one.

 

Better Maps Make Safer Oceans

 

The applications are real and unusually varied. Navigation safety depends on knowing where the shallow bits are, and uncharted seamounts have damaged submarines and run ships aground. Submarine cable routing, which determines the resilience of global data traffic, requires detailed knowledge of slope stability, sediment type and seismic hazard along thousands of kilometres of seabed. Tsunami modelling is highly sensitive to bathymetry, because the shape of the seafloor determines how a wave propagates and where it amplifies. Fisheries management depends on knowing where the habitat is. Offshore wind foundations, oil and gas infrastructure, and marine protected area design all need the same underlying data. So does climate modelling, since seafloor topography steers deep ocean circulation.

There is a tension in that list worth naming rather than glossing. The same map that enables a marine protected area to be drawn around a seamount enables that seamount to be assessed for mining. Better bathymetry serves conservation and extraction equally well, because both require knowing what is there. Seabed 2030's data is open, which is the right decision, and it means the map is a public good that will be used for purposes its contributors disagree about.

Which is the honest version of the carousel's closing line. Understanding the ocean floor is essential to using it responsibly, and the map itself is neutral about what responsible means. Having the map is the precondition for the argument, not the resolution of it.

 

Note: This article reflects the state of seabed mapping as of October 2026, drawing on sources including the Nippon Foundation GEBCO Seabed 2030 project's 2026 grid release, published research on SWOT satellite altimetry and marine gravity-derived bathymetry, and UNESCO Intergovernmental Oceanographic Commission statements on mapping targets and costs. The completion timeline calculated in this article is the author's arithmetic from Seabed 2030's published coverage and annual addition figures, and assumes the current rate holds, which the project does not claim.