For most of human history, the sea floor was less well understood than the surface of the moon. A sailor sounding with a lead line could measure the depth of a shallow harbour, but two-thirds of the world's oceans are deeper than three thousand metres, and a length of weighted rope dropped into such water tells almost nothing about the shape of the bottom. Modern bathymetry - the mapping of underwater topography - emerged only in the mid-twentieth century, when echo-sounding technologies developed for anti-submarine warfare were turned to scientific use.

The basic idea is simple. A ship emits a pulse of sound downward; the pulse reflects off the sea floor and returns to a microphone on the hull; and the time between emission and return, multiplied by the speed of sound in water, gives twice the depth. Early single-beam echo sounders produced a single depth reading every few seconds along the ship's track. By the 1970s, multi-beam systems could produce a fan of readings across a strip of seabed several kilometres wide, and computers could assemble the readings into a contoured map. The technique revealed features of the ocean floor that had been entirely unsuspected, including mid-ocean ridges, abyssal trenches, and the vast submarine landslides that mark the edges of many continental slopes.

The pattern of ridges was the most consequential of the discoveries. A continuous underwater mountain chain, running for more than sixty thousand kilometres around the globe, had been suspected from scattered soundings in the early twentieth century. Systematic bathymetric mapping in the 1950s and 1960s revealed its full extent and the characteristic rift valley that runs along its crest. The British geophysicist Linda Carter has described the result as "the most important map made in the twentieth century", because the ridge system provided the critical geometric evidence for the theory of plate tectonics. Without a complete picture of the ridges, the idea that the ocean floors are spreading outward from them, and that the continents are passengers on the moving plates, would have been much harder to accept.

Despite these advances, most of the sea floor remains poorly mapped. Multi-beam surveys are slow and expensive, and coverage has historically been concentrated along shipping routes and in the exclusive economic zones of wealthy nations. As of 2024, a detailed map has been produced for perhaps twenty-five percent of the ocean floor; for the remaining three-quarters, the best available data come from satellite altimetry, which measures the small variations in sea-surface height caused by the gravitational pull of large underwater features. Satellite maps can detect any seamount taller than about a kilometre, but their horizontal resolution is too coarse for most practical purposes.

An international effort called Seabed 2030, coordinated by the British institution IHO and the Nippon Foundation, aims to produce a complete high-resolution bathymetric map of the world's oceans by the target date. The project relies in part on donated survey data from commercial shipping and research fleets, and in part on the development of autonomous survey vessels that can operate for weeks at a time without a crew. The British engineer Fiona Marsh, who leads one of the autonomous-vessel programmes, has argued that the cost of surveying a square kilometre has fallen by almost an order of magnitude over the past decade, making the target technically plausible if the political will to continue funding the effort can be sustained.

The practical applications of better maps are wide. Seabed cables, which carry most intercontinental internet traffic, cannot be laid safely without detailed knowledge of the terrain; a cable routed over an unmapped underwater mountain risks being damaged by abrasion, and a cable laid across an active sediment slope risks being severed by a submarine landslide. Fishing industries depend on knowing the topography of the continental shelf to locate productive banks and to avoid damaging fragile deep-sea corals. Renewable-energy developers locating offshore wind or tidal installations need bathymetric detail that was simply unavailable a generation ago.

Scientific applications are as important as the commercial ones. Climate modelling requires accurate bathymetry because the flow of deep ocean currents - which carry heat around the planet - is controlled by the shape of the sea floor. A twenty-kilometre error in the position of a submarine ridge can produce substantial errors in the simulated flow of water across it, and in turn in the simulated distribution of temperatures at the surface. Marsh has remarked that her fieldwork is sometimes popularly described as surveying the last unknown part of the Earth, but that the description understates the importance of the activity: "We are not filling in an incidental detail", she wrote in a 2022 report. "We are providing a basic input without which much of modern earth science is simply guessing."

The challenges are not only technical. Maps of the sea floor reveal features, including potential mineral deposits, that have become the subject of competing national claims. The International Seabed Authority, established under the United Nations Convention on the Law of the Sea, has the responsibility of regulating mineral exploitation in areas beyond national jurisdiction, but its procedures have struggled to keep pace with industry interest. Environmental scientists worry that deep-sea mining, once begun on a commercial scale, could damage ecosystems that took thousands of years to develop. Carter has called for detailed bathymetric mapping to precede any decision about exploitation, on the argument that decisions about the deep ocean should not be made without a proper view of what lies below.