IELTS Reading: Short answer

It is often said that the surface of Mars is better mapped than the floor of the ocean, and the claim, though it sounds like the sort of thing said for effect, is broadly accurate. Orbiting spacecraft have produced a global picture of Mars at a resolution of a few tens of metres, because a camera in orbit can see the whole planet and nothing stands in the way. Light does not travel through four kilometres of seawater, and so nothing about the deep sea floor can be seen from above at all. Everything we know about its shape has had to be measured by other means, and every one of those means is either slow or coarse.

For most of the history of navigation there was only one method. A weight on a marked line was lowered over the side until it touched bottom and the line was measured. In shallow water this is quick. In deep water it is an operation lasting hours, in which the ship must stay in one place, the line must be kept vertical against the current, and the moment of contact must be judged by the feel of a rope several kilometres long. The great British oceanographic expedition of the 1870s spent three and a half years at sea and returned with fewer than five hundred deep soundings, which was considered a triumph and was, for its time.

Sound changed everything. A pulse directed downwards returns as an echo, and since the speed of sound in seawater is known to within a fraction of a per cent, the time it takes gives the depth. Echo sounders were in commercial use by the 1920s, producing a continuous line of depths along a ship's track instead of isolated points. The decisive improvement came later, with systems that send out a fan of beams rather than a single pulse and so measure a whole strip of sea floor at once. The width of that strip is roughly four times the water depth, which means a single vessel in deep water can chart a band some sixteen kilometres across as it steams.

Sixteen kilometres sounds a great deal until it is set against the size of the job. The ocean floor covers some three hundred and sixty million square kilometres. A ship working continuously, never stopping for weather, fuel or crew, would need centuries to cover it, and no ship works continuously: a vessel capable of this kind of survey costs in the region of fifty thousand dollars a day to operate, and its time is allocated to projects that have specific reasons for wanting specific places. Systematic mapping of empty ocean has never had a sponsor.

There is a second method, and it explains why maps of the whole sea floor nevertheless exist. A satellite can measure the height of the sea surface to within a few centimetres. That surface is not flat: a large underwater mountain has enough mass to attract the water above it, raising a bump at the surface a metre or two high and several kilometres across. Read backwards, the shape of the sea surface therefore reveals the shape of the sea floor beneath it. The method covers the entire globe in a matter of weeks and resolves features down to a kilometre or two across. Anything smaller than that, which includes most of what matters to a submarine cable or a fishing fleet, is invisible to it.

The uses of better charts are not academic. The height a tsunami reaches when it strikes a coast depends on the shape of the sea floor it crosses, and modelling of tsunamis is only as good as the depth data fed into it. Cable companies need to know what their cable will lie on. Fisheries management depends on knowing where the seamounts are, because that is where the fish congregate. When an airliner disappeared over the southern Indian Ocean in 2014, the search vessels sent to look for it surveyed about two hundred and eighty thousand square kilometres of sea floor that nobody had ever mapped, and found, among other things, two uncharted volcanoes.

An international project now aims to produce a complete map of the ocean floor to an agreed standard by the end of this decade. Its method is partly to coordinate existing surveys and partly to gather data that would otherwise be thrown away: merchant ships, research vessels and even well-equipped yachts crossing the ocean carry echo sounders whose records are of no further use to them and of considerable use to a chart. Coverage has risen from about six per cent when the project began to roughly a quarter now, which is remarkable progress and still leaves three quarters of the sea floor known only from the shape of the water above it.