Mapping the Ocean Floor

A Until surprisingly recently, the floor of the deep ocean was less well known than the surface of the Moon. For thousands of years the only way to measure the depth of the sea was to lower a weighted rope over the side of a ship and wait for it to touch the bottom, a slow technique called sounding. Each cast produced a single number at a single spot, and in deep water one measurement could take hours as the line was paid out and hauled back in. Sailors built up their charts one laborious sounding at a time, and vast stretches of the ocean floor remained entirely blank, imagined rather than mapped. A captain might know the depth beneath his own keel at a single instant, yet have not the faintest idea what lay a mile ahead, and wrecks on uncharted shoals were a constant hazard of long voyages.

B The first attempt to survey the deep sea across the whole globe was the voyage of HMS Challenger, which sailed from England in 1872 and did not return until 1876. Over three and a half years its crew lowered their lines many hundreds of times, recording depths in every ocean they crossed and hauling up mud and unfamiliar creatures from the bottom. Among their soundings was an astonishingly deep trench in the western Pacific, the deepest point then known to science. The expedition is widely regarded as the moment the modern study of the oceans began, and it kept scientists busy analysing its samples for decades afterwards. The Challenger carried a team of naturalists as well as sailors, and the reports they eventually published filled dozens of thick volumes, describing hundreds of creatures never seen before and laying the foundations of the science of oceanography.

C Everything changed after the First World War with the arrival of echo sounding. Instead of a rope, a ship now sent a pulse of sound down towards the seabed and measured the time the echo took to return; because the speed of sound in water is known, that time could be turned directly into a depth. Readings that had once taken hours could be made in a matter of seconds, and a vessel steaming along could trace a continuous line of the bottom beneath it. For the first time the ocean floor could be drawn as a landscape, with slopes and ridges, rather than as a scatter of isolated dots. Warships and survey vessels alike were soon fitted with the new devices, and over the following decades the outlines of features no one had suspected, deep trenches, broad plains and towering underwater peaks, began to emerge from the once-featureless dark.

D In the middle of the twentieth century two American scientists, Marie Tharp and Bruce Heezen, set about turning columns of echo-sounding figures into a picture. Working mostly with data gathered in the Atlantic, Tharp plotted profile after profile by hand and noticed a long undersea mountain range running down the very middle of the ocean. More striking still, a deep valley ran along the crest of this range, like a groove cut into its spine from end to end. At first she could scarcely believe her own plots, and she checked them over and over, but the valley refused to go away.

E Tharp suspected that the central valley was a place where the sea floor was slowly splitting apart, an idea that fitted the then-controversial notion that the continents drift. At first her interpretation was brushed aside; Heezen himself is said to have dismissed it as mere 'girl talk'. Yet as more of the world's oceans were surveyed, the same ridge-and-rift pattern turned up everywhere, encircling the globe like the seam on a baseball. Her reading of the data proved correct and became one of the foundations of the theory of plate tectonics.

F Mapping has since moved into space. Satellites cannot see the sea floor directly, but they can measure the height of the sea surface with great precision, and that surface is not perfectly flat: the gravity of an undersea mountain pulls a little extra water into a slight bump above it. From these tiny bumps the shape of the hidden floor can be worked out. Even so, most of the ocean bottom has still been charted only at coarse resolution, and an international effort called Seabed 2030 is now racing to produce a detailed map of the entire ocean floor before the decade is out. Knowing the true shape of the sea floor is far from an idle curiosity: it sharpens forecasts of how tsunamis will travel, guides the laying of the cables that carry the internet between continents, and helps scientists understand the deep currents that shape the world's climate.