The oceans that cover seven tenths of the world’s surface present a unique challenge to map makers. There are no roads, rivers, cities and towns to chart, and to give a sense of scale and distance. Such features as there are – winds, currents, tides – are intangible and forever on the move.
For centuries, the sea was shown on maps as a blank space between landmasses, which cartographers decorated with fantastic monsters to make up for the absence of other detail. What lay beneath the waves was a mystery, and even today, only 20 per cent of the ocean floor has been mapped.
The Carta Marina (1539) by the Swedish topographer Olaus Magnus fills the otherwise empty ocean with strange creatures.
Here be monsters
The earliest navigational maps were the stick charts made by the Polynesians, who tied together lengths of bamboo or other wood, marking the locations of islands with shells or knots. Curved pieces of wood represented the movement of the waves around the islands, and the effect of the waves on their canoes.
The sea charts known as portolans used by European and Arab sailors in the Middle Ages are actually maps of the coasts. These were shown in great detail, with every port, headland and bay depicted and named. But the open sea remained a void, criss-crossed with rhumbs – diagonal lines emanating from the 32 points of the compass to enable mariners to chart a course to their destination. Islands were shown, but with no accurate way of measuring longitude, their east-west placing was haphazard.
Portolan chart by Jorge de Aguiar (1492) (Beinecke Rare Book and Manuscript Library, Yale University).
In the 16th century, navigators began measuring the depths of coastal waters, estuaries and harbours by lowering a weighted line over the side of a boat or, in shallows, using poles. These soundings were plotted on to sea charts such as Lucas Janszoon Waghenaer’s famous Spieghel der Zeevaerd (The Mariner’s Mirror) of 1584 to prevent sailors from running aground on sandbanks and shallows.
The Thames Estuary, from Lukas Janszoon Waghenaer, Spieghel der Zeevaerd ,1584. Waghenaer’s chart extends from Dover in Kent to Orford Ness in Suffolk, with north at the right. The numbers indicate depth in fathoms; the sandbanks, mapped in detail, have shifted in the intervening centuries.
By the beginning of the 19th century, the offshore waters of Europe and North America had been thoroughly sounded, but beyond the continental shelf, where it was too deep to drop a plumb line, there was no way of measuring the sea floor; and because there was no danger of ships running aground, there was little incentive to do so. What happened below the open sea remained as much a mystery as when Magellan dropped a rope from his ship and, finding that it did not reach the bottom, concluded that the depth of the ocean was infinite.
The Scientific and Industrial Revolutions
It was the scientific advances of the 18th and 19th centuries that made it possible to map the oceans more thoroughly, and the burgeoning trade and industry of the period that made it necessary.
The first reliable deep-water sounding was made by the British naval officer James Clark Ross on his Antarctic expedition of 1839-43. Ross pushed the traditional rope method to its limits to plumb the South Atlantic to a depth of 2425 fathoms (4365m).
Before long, though, the development of sounding machines, using reels to spool out a wire and measure its length, made systematic deep-sea soundings more practicable, and gave birth to the science of bathymetry – the mapping of the ocean floor.
Bathymetric map of the Atlantic, from The Physical Geography of the Sea (1855) by M. F. Maury.
Mapping the ocean floor
Telecommunications – one of the main drivers of oceanographic research today – entered the picture in 1858, when the first submarine telegraph cable was laid from Ireland to Newfoundland. Charting the seabed was no longer a matter of scientific curiosity alone – it had immediate practical applications.
The route of the 1858 telegraph cable, from Howe’s Adventures and Achievements of Americans. The vignette below shows the profile of the seabed as plotted by the SS Arctic.
In 1871, the British government sponsored the Challenger expedition to research the salinity and temperature of seawater, ocean currents, and underwater mountain chains. The expedition travelled nearly 130,000km and sounded far deeper than anyone had done before, reaching 5700m at Challenger Deep in the Marianas Trench. The resulting data filled 50 volumes of reports, and astounded the public, revealing a hidden world:
“The bottom of the ocean it appears is as varied as the land for there are valleys & mountains, hills & plains all across the Atlantic.”
In January 1874, the USS Tuscarora took soundings for a submarine cable between the United States, Japan, and China, while in 1891 the Albatross, an iron-hulled, twin-screw steamer that was reputedly the first vessel built specifically for marine research, set out to determine “a practicable route for a telegraphic cable” between San Francisco and Honolulu.
"Albatross-ii" by Unknown - NH 91740, U.S. Army History Institute, Carlisle Barracks, Pennsylvania. via Wikimedia Commons
By the close of the 19th century, almost all of the world’s coastlines, except for parts of the polar regions, had been charted in detail. Oceanography was an established science, and a far-reaching infrastructure of shipping lines and telegraph cables spanned the globe.
During the First World War, the British Anti-Submarine Detection Investigation Committee (ASDICS) developed a system that transmitted sound waves underwater and used their echoes to locate submerged objects and measure distances. During the Second World War it acquired the name sonar (SOund, NAvigation and Ranging), by which it is now generally known. As well as finding subs, the technology offered an easier and more reliable method of charting the ocean floor than the old method of dropping a weighted line overboard.
The satellite age
Navigation on land, at sea and in the air was revolutionised once again by the space race of the 1960s. The US military developed a Global Positioning System (GPS), launching its first GPS satellite in 1978.
Satellite technology gave rise to the science of marine geodesy. It is now understood that gravity causes the ocean's surface to bulge outward and inward, mimicking the topography of the ocean floor. Using satellites to measure these bulges, it is now possible to construct a model of the ridges and troughs that lie beneath the waves. Harnessing gravity measurements from two orbiting satellites, the Scripps Institution of Oceanography in California has created a new map of the ocean floor that reveals thousands of previously uncharted mountains rising from the deep.