Mapping the Seafloor

We have better maps of the surface of Mars than of the floor of our own oceans. The statement is repeated often enough to have become a cliché, but it is broadly accurate. Satellites can photograph a planetary surface directly; water blocks the instruments that make such imaging possible, so the seafloor must be measured by other means.

The earliest method was simple and slow. A weighted line was lowered until it touched bottom, and the length paid out was recorded. A single reading might take an hour. The survey ship Challenger, on its voyage of 1872 to 1876, took around five hundred such soundings in three and a half years — an extraordinary achievement at the time, and a negligible sample of an ocean floor covering more than three hundred million square kilometres.

Echo sounding, developed in the early twentieth century, transformed the picture. A pulse of sound is emitted and the time taken for its echo to return is measured; since the speed of sound in seawater is known, the depth follows. Early devices produced a single depth reading beneath the ship. Modern multibeam systems emit a fan of pulses and map a swath of seabed hundreds of metres wide as the vessel moves.

Even so, ship-based surveying is slow. Covering the entire ocean floor at high resolution by this method would occupy a large fleet for decades. This is why most existing global maps rely instead on satellite altimetry, which measures the height of the sea surface with great precision. The connection is indirect but reliable: a large seamount exerts a slight additional gravitational pull, drawing water towards it and raising the sea surface above by a small amount. Reading the surface therefore reveals the shape of the floor beneath. The resolution is coarse — features smaller than a kilometre or so are invisible — but the coverage is global.

The consequences of poor mapping are not merely academic. Undersea cables carry almost all international data traffic, and route planning depends on knowing the terrain. In 2005 a United States submarine struck an uncharted seamount, killing one crew member. Tsunami models, which predict how a wave will travel and where it will come ashore, are only as good as the bathymetric data fed into them.

An international project launched in 2017 aims to map the entire ocean floor at high resolution by 2030. Progress has been faster than many expected, partly because commercial vessels have been persuaded to share data they collect routinely. Whether the target is met may depend less on technology than on whether such data continues to be shared.