Why Have We Mapped More of Mars Than the Ocean Floor?

Background

Robotic orbiters have imaged the entire surface of Mars in enough detail to track individual sand dunes shifting between seasons, and NASA's Mars Reconnaissance Orbiter can resolve targeted areas down to about 25 centimetres. Venus, permanently shrouded in thick cloud, has been comprehensively radar-mapped since NASA's Magellan mission in the early 1990s. Even the Moon has been laser-altimetry mapped to a resolution finer than most of Earth's own seafloor. Meanwhile, as of 2026 the international Nippon Foundation-GEBCO Seabed 2030 project, which coordinates and compiles bathymetric survey data from research institutions, navies, and commercial vessels worldwide, reports that only about 28.7% of Earth's ocean floor has been mapped to a modern, genuinely useful resolution, an improvement from 27.3% the year before, gained by adding nearly five million square kilometres of new survey data in a single year. Since oceans cover roughly 71% of the planet's surface, that means the majority of Earth's own seabed remains less well characterised than the surface of a planet 225 million kilometres away on average.

Why Orbit Works for Planets but Not for Oceans

The explanation is a straightforward matter of physics rather than a gap in scientific priorities. Spacecraft map planetary surfaces using radar and laser altimeters, instruments that send out a pulse and measure how long it takes to bounce back; because Mars's atmosphere is thin and Venus's, while thick, is still largely transparent to the right radar wavelengths, these signals travel from orbit to the ground and back with only modest interference, letting a single spacecraft methodically cover an entire planet over a period of months to years. Seawater is a fundamentally different medium for this purpose: it absorbs radar and laser wavelengths within centimetres to, at best, a few tens of metres, nowhere close to the roughly 3,700-metre average depth of the world's oceans. No orbiting instrument can currently see through that much water to the seabed beneath.

Sound waves, not light, are the signal that actually travels efficiently through seawater, which is why every genuinely detailed ocean floor map depends on sonar rather than any remote-sensing technique available from orbit. Ships equipped with multibeam sonar systems sail back and forth across the ocean, each pass mapping a swath of seafloor a few times wider than the water is deep directly beneath the vessel, building up a detailed picture one narrow strip at a time. Covering the entire ocean this way is a genuinely enormous undertaking: it requires an immense number of ship-hours across some of the most remote parts of the planet, is expensive per square kilometre compared with a single orbiting spacecraft's coverage, and competes for vessel time with commercial shipping, fishing, naval, and other research priorities that rarely align with systematic seafloor survey.

The Coarser Map That Already Exists

Direct sonar survey is not the only bathymetric data available, and it is worth being precise about what "28.7% mapped" actually means. Since the 1990s, oceanographers including David Sandwell and Walter Smith have built global seafloor models by an indirect method: satellite radar altimeters measure tiny variations in sea-surface height, on the order of centimetres, caused by the gravitational pull of undersea mass, a seamount subtly raises the sea surface above it, a deep trench subtly lowers it, and from this signal researchers can infer the broad shape of the seafloor everywhere the satellites have flown, which is effectively the whole ocean. This method has produced a genuinely global bathymetry map for decades. Its resolution, however, is only a few kilometres, useful for revealing the existence and rough shape of major features like seamount chains and trench systems, but far too coarse to resolve anything smaller than a large hill, to support safe navigation, or to serve the marine biology, geohazard, and engineering applications that need metre-scale detail. Seabed 2030's percentage figure tracks the much harder, ship-based work of replacing that coarse backdrop with genuinely detailed direct measurement.

What Better Maps Have Already Found

The value of closing this gap shows up concretely every time new survey lines cross previously unmapped ocean. Thousands of seamounts, some rising more than a kilometre from the surrounding seabed, have been newly catalogued simply because a ship happened to cross terrain no sonar had covered before; features of that size are easily smoothed away by the coarser satellite-gravity method but unmistakable once direct sonar passes overhead. Beyond individual features, detailed mapping has revealed submarine canyon systems, fault and fracture zones relevant to tsunami-hazard modelling, safer routes for the undersea cables that carry the bulk of global internet traffic, and habitats that turn out to matter directly for biology: the 1977 discovery of thriving ecosystems clustered around deep hydrothermal vents, organisms living on chemical energy in total darkness rather than sunlight, came from exploration of terrain that detailed mapping had only just made accessible to study, and remains the clearest demonstration that Earth's own seafloor can still deliver genuine scientific surprises on the same order as anything found on another planet.

Current Consensus

There is no serious dispute about why this gap exists: the physics of how radar and sonar behave in water versus vacuum or thin atmosphere, combined with the sheer scale and remoteness of the ocean relative to available ship time and funding, fully accounts for it, without needing to invoke any lack of scientific interest in the seafloor. What remains open is a matter of pace and priority rather than explanation: Seabed 2030 aims to bring direct high-resolution coverage close to complete by 2030, a target the project's own recent progress, adding several million square kilometres a year, suggests is achievable only if survey effort continues to accelerate, and crowdsourced data from commercial and naval vessels, not just dedicated research cruises, keeps expanding at the current rate. That same unmapped seafloor is also where the disputed 2024 "dark oxygen" nodule claim originated, a reminder that better maps alone won't settle every open question about what the deep ocean is actually doing.

Why This Mystery Endures

The Mars-versus-ocean-floor comparison endures because it is one of the rare, genuinely accurate ways to communicate how little of Earth's own seabed anyone has actually looked at closely, without exaggeration or speculation. Unlike many "we know so little about the ocean" claims that get repeated past the point of accuracy, this one holds up on inspection: the coarse global gravity-derived map has existed for decades, but the useful, ship-based, high-resolution picture genuinely does lag behind planetary mapping achieved from orbit, and the reason is a clean, checkable fact about how radar and sonar behave in different media rather than a vague appeal to how vast or mysterious the ocean is. That gives the comparison unusual staying power as a talking point: it is simultaneously startling and, once explained, completely mundane, exactly the combination that tends to travel well.

It also functions as the natural starting point for this site's other ocean-depth coverage, because it explains a structural fact underlying most of it: why a case like what actually lives at the bottom of the Mariana Trench required a small number of extraordinarily expensive dedicated expeditions rather than routine survey, why an oddly-shaped sonar return like the Baltic Sea anomaly or the Yonaguni Monument can circulate for years before enough direct data accumulates to settle what it actually is, and why entire ecosystems, like the vent communities first found in 1977, can still turn up unannounced in terrain nobody had previously mapped in detail. The ocean floor is not, and never was, an unknowable void; it is simply the part of the planet that has to be measured the slow way, one sonar swath at a time, and the Mars comparison is the clearest single number that conveys exactly how much of that slow work still remains.