Measuring the unseen
For most of maritime history, sailors cared about depth only near land. A hand lead on a line, dropped from the chains of a ship, told a pilot how much water lay under the keel and, with tallow in a hollow in the lead, what the bottom was made of. Open-ocean depths were largely guesses, and some early attempts to sound the deep sea failed because a weight on a long line kept running out without anyone knowing when it touched bottom.
From the middle of the nineteenth century, the need to lay telegraph cables across the Atlantic pushed navies and scientists to find out what lay on the seabed. This gave rise to ships built or fitted specifically for scientific work.
Early deep-sea sounding
By the 1850s, deep-sea sounding used a weighted line, sometimes of hemp and later of wire, with a device that released the weight at the bottom so the line could be hauled up more easily. Survey officers in the Royal Navy and the United States Navy collected soundings across the North Atlantic. These suggested that the ocean floor was not a flat plain, and led to the idea of a raised area in the middle of the Atlantic, then often called the telegraph plateau.
Sounding by line was slow. A single measurement in deep water could take hours, so the number of readings that a ship could make in a year was small. This is one reason the full picture of the ocean floor developed gradually.
From Challenger to the specialist ship
The voyage of HMS Challenger in the 1870s was the first large, systematic oceanographic voyage, and its methods are described in the article on the Challenger expedition. It used a converted naval corvette. Purpose-built research vessels came later, when governments and institutions saw value in dedicated ships.
Examples of the early twentieth century included the Albatross, an American fisheries and research steamer, and the yachts of Prince Albert I of Monaco, who funded oceanographic cruises. Britain’s Discovery Investigations in the 1920s and 1930s used the ship Discovery and a smaller vessel, William Scoresby, to study whales and Southern Ocean conditions. These voyages fed into the knowledge needed by the whaling industry, a link explored in the article on Antarctic whaling.
The echo sounder
The most important technical change was the echo sounder. It sends a sound pulse from the ship’s hull and measures the time taken for the echo to return from the seabed. Knowing the speed of sound in water, the operator can calculate depth. Development of practical systems in the early 1920s made it possible to take continuous soundings while a ship was steaming along.
The German research ship Meteor, on her expedition in the South Atlantic in 1925 to 1927, is widely cited as a pioneer of this method. Her crew took very large numbers of echo soundings along many tracks across the ocean, which showed the seabed to be a landscape with ridges, basins and mountains. It was a leap beyond what lead lines could provide.
Mapping the mid-ocean ridge
After the Second World War, American research institutions expanded their fleets. At Columbia University’s Lamont laboratory, the scientists Marie Tharp and Bruce Heezen used soundings from ships such as Vema, along with data from other vessels, to draw maps of the ocean floor in the 1950s. Tharp’s drawings brought out a continuous ridge system running through the oceans, with a central valley in places. These maps helped to support the theory of plate tectonics in the following decade.
Sonar technology improved over this period. Side-scan sonar produced images of the seabed to either side of a ship’s track, and multibeam echo sounders, widely used from the later twentieth century, could measure a broad swath of depths at once, sharply raising the efficiency of surveying.
Currents, water and sediment
Depth was not the only quantity measured. Research ships also took water samples at set depths using bottles that closed on command, measured temperature and salinity, and tracked currents with drifting floats and later with acoustic instruments. Cores of sediment were pulled up in long tubes, providing evidence of past climates and the age of the seafloor.
Ship design adapted to the work. Research ships typically have large open working decks, powerful winches with long cables, A-frames and cranes for lowering equipment, wet and dry laboratories, and dynamic positioning or bow thrusters so that they can hold a position over a point for hours. Hulls are designed to reduce noise so that the echo sounders and hydrophones work well.
Drilling and submersibles
In the 1960s the idea of drilling into the ocean floor produced the drillship Glomar Challenger, which began work for the Deep Sea Drilling Project in 1968. The ship used a dynamic positioning system to stay over a hole in deep water. Cores recovered from the seabed supported the case that the ocean floor was spreading outward from ridges.
Crewed submersibles were also launched from support ships. The Alvin, first operated in 1964, became a well-known example, and with its mother ships it later explored hydrothermal vents and the wreck of the Titanic. Many such operations rely on remotely operated vehicles tethered to ships in recent decades.
A map that is still being finished
Large parts of the ocean floor have still been mapped only at low resolution, and some are known mostly from satellite measurements of the sea surface, which reflect the shape of the seabed below. Research ships remain essential for detailed surveys. Today’s international projects aim to compile a high-resolution map of the whole seabed, but that work is incomplete and depends heavily on ship time.