A navy ship turned into a laboratory
In December 1872 HMS Challenger left Portsmouth on a voyage that lasted until May 1876. It is usually regarded as the first major expedition devoted to studying the deep sea as a whole, and it is often said to have given birth to oceanography as a science. That claim is a simplification, since earlier surveys and soundings existed, but the scale and organisation of the Challenger work were new.
The expedition was proposed by scientists of the Royal Society, notably William Carpenter and Charles Wyville Thomson, who had seen unexpected animals dredged from deep water in the 1860s. The Admiralty supplied the ship and the funds.
The ship and the people
Challenger was a wooden steam-assisted corvette. She retained her sails for most of the voyage and used her engine for sounding, dredging and manoeuvring. Most of her guns were removed to make room for laboratories, a chemistry space, storerooms for specimens and a dredging platform. The naval captain was George Nares at first; he left the ship during the voyage to lead a separate Arctic expedition, and Frank Tourle Thomson took over.
The scientific staff was led by Wyville Thomson, a professor of natural history at Edinburgh. His colleagues included the naturalists John Murray, Henry Moseley and Rudolf von Willemoes-Suhm, the chemist John Young Buchanan and the artist John James Wild. Willemoes-Suhm died of illness during the voyage in 1875. The expedition was a naval and scientific partnership at a time when the steamship era was making long voyages more predictable.
Where the ship went
Challenger crossed the Atlantic several times and visited the Cape, the southern Indian Ocean, Australia, New Zealand and parts of the western Pacific, including the Philippines and Japan. She crossed the Pacific to South America and returned via the South Atlantic. The ship covered some 68,000 nautical miles, a figure often quoted, and stopped at several hundred stations to measure and sample.
She also crossed the Antarctic Circle in February 1874, and was the first steamship known to have done so. She did not see the continent, and the ice limited her southward progress.
What was measured
At each station the staff attempted a standard set of observations. They sounded the depth by lowering a weighted line, which could take a long time in deep water. They collected samples of the seabed with the sounding apparatus and dredged or trawled to bring up animals living on or near the bottom. Temperatures were measured at different depths with special thermometers designed to resist pressure. Water was sampled for salinity and gases, and surface and deep currents were assessed.
The most striking finding in the first years was that life existed at all depths, including those where it had been widely believed there was none. Another was that the temperature of deep water was very low everywhere, even in the tropics, which pointed to the movement of cold water from the poles.
Discoveries
The expedition identified thousands of previously unknown species of marine animals, a figure often given as roughly 4,700 new species, though counts vary. Sampling of the seabed showed widespread deposits of ooze made of the shells of tiny organisms, and fields of manganese nodules on the ocean floor.
Soundings helped to reveal the shape of the ocean basins. A sounding in the western Pacific, near the Mariana Islands, found a depth of around 4,500 fathoms, which is more than 8,000 metres, in what later became known as the Mariana Trench. The deepest area of that trench is still called the Challenger Deep, after the ship. Soundings in the Atlantic suggested a raised ridge down the middle of the ocean, an idea that later research ships would confirm and map in detail.
The report
After the ship returned, Wyville Thomson began organising the findings, but he died in 1882. John Murray took over as editor and the Challenger Report was eventually published in about fifty large volumes by 1895. It covered zoology, botany, physics, chemistry and geology and remained a standard source for decades.
How it differed from other survey voyages
Earlier voyages had mixed science with exploration. Cook’s Pacific voyages, for example, combined coastal survey with natural history, and the Beagle survey voyages of the 1830s had gathered geological and biological data while charting South America. Challenger was different in being primarily devoted to the deep ocean, with a specialist team and a systematic plan of stations.
Legacy
The cruise established ocean science as a field with its own methods and institutions, and later nations followed with their own expeditions. Techniques such as reversing thermometers, sediment sampling and standardised stations became part of normal practice. Modern oceanographic research vessels still follow the same basic cycle of stations, measurements and samples, but with echo sounders, satellites and electronics replacing hemp and wire.
Challenger herself was paid off after the voyage and later served as a receiving hulk and a depot ship before being broken up in the early twentieth century.