The history of deep sea exploration:
The Challenger Expedition, 1872-1876

Throughout the ages, man has been driven by a desire to explore our world, but exploration of the deep ocean is a relatively new science.

Serious investigation of the deep sea environment didn’t take place until the 19th Century when the historic HMS Challenger expedition (1872 – 1876) set out to discover what lay beneath the surface of the world’s oceans. Prior to this voyage, the small amount of knowledge about the ocean floor came from information gathered by the Navy. However, after the first submarine telegraph cable was successfully laid across the English Channel in 1851, governments and cable companies quickly realised that much more knowledge about the seafloor was needed. The new boom in telegraph communication meant that knowledge of the seafloor environment was vital to ensure that expensive communication cables could be laid down safely, without the risk of them being broken or lost.

But interest in the oceans wasn’t just restricted to cable companies. Since Charles Darwin published his 'Origin of the Species' in 1859, fossil collecting on beaches had become a hugely popular pastime among the Victorian public. As the search for fossils extended from the beach into the shallow sea, so the public’s interest in what lay beneath the water increased. Fossil hunting began to involve casting out nets from small boats to collect samples from the seafloor, and as this progressed biologists began to discover new and exciting modern species of animals living in marine environments which were previously believed to be too cold and dark to support life. These new discoveries, combined with new observations about ocean currents, were the driving force behind the HMS Challenger voyage, which was the first ever dedicated marine science expedition.

The Challenger Expedition, 1872-1876

The first marine science research cruise departed from Portsmouth on 7th December 1872, bound for a three and a half year-long journey all around the world’s oceans. Aboard HMS Challenger were a team of 6 scientists led by British biologist Charles Wyville Thomson, 20 Navy officers and a crew of more than 200 men. The Challenger expedition marked the start of a new era in marine exploration.

HMS Challenger

HMS Challenger was loaned to the scientific mission by the British Navy. In order to be suitable for carrying out scientific investigations, the ship had to undergo extensive alterations, including removal of all but two of the cannons to make room for equipment and sample storage. 144 miles of hemp rope (used for sounding) and 12.5 miles of piano wire (used for pulling up samples from the seabed) were loaded onto the ship.
In addition, two laboratories were constructed on board – one for chemistry and one for biology (shown below).

The chemistry lab

The natural history lab

The sampling deck

During her journey, HMS Challenger travelled almost 69,000 nautical miles (130,000 km), and visited every continent on the planet, including Antarctica. The ship stopped for sampling at 362 sample stations, spaced as regularly as possible over the ocean floor. A strict sampling routine was followed at each station, and the following measurements were made:

The exact depth was determined by lowering a 100lb weight over the side of the ship on hemp rope until it touched the bottom (a process known as sounding). This process could take a whole day just for one depth measurement.
A sample of the seafloor using the same equipment used for sounding.
A sample of bottom water was taken for chemical/physical examination.
The bottom temperature was recorded by a thermometer.
At most stations, a fair sample of the biology lving on or just above the seafloor was taken using a dredge or trawl net.
At most stations, the biology of the surface and of intermediate depths was examined using towed nets.
At most stations, a series of water temperature observations were made at different depths from the surface to the bottom.
At many stations, samples of seawater were obtained from different depths.
In all cases, atmospheric and other meteorological (weather) conditions were carefully observed and noted.
The direction and rate of the surface current was measured.
At a few stations, an attempt was made to measure the direction and rate of movement of the water at different depths.

The main focus of the expedition was marine biology. 4714 new species of marine animals were discovered during the course of the voyage, many of which were found on the seafloor - an environment that scientists originally believed to be too inhospitable to support life. This deep-sea sponge (left) and the dinoflagellates (below) are examples of some of the creatures that were discovered. All the new species were carefully described, and many were sketched by the expedition's artist, J.J. Wild.

Euplectella subearea

However, some of the greatest discoveries arose from measurements of the temperature and salinity (saltiness) of seawater deep in the ocean. These measurements led to the discovery of ocean circulation. Previously, it was believed that limited movement of water around the ocean was caused by surface winds creating currents in the sea. However, measurements taken by scientists on board HMS Challenger showed that differences in the temperature and salinity (and therefore the density) of different water masses were the driving force behind ocean circulation.

Another major discovery of the Challenger expedition involved the Mid-Atlantic Ridge. Early measurements of the depth of the ocean by cable surveyors had revealed that the deepest part of the North Atlantic was not in the centre, as expected, but either side of a linear ridge-like structure which ran roughly north-south down the centre of the ocean. Scientists aboard HMS Challenger used soundings to prove that this ridge continued southwards into the South Atlantic Ocean, and appeared to ‘branch off’ in several places. The geological significance of this structure remained a mystery until the 1960’s, when plate tectonic theory was developed and it became accepted that mid-ocean ridges like the Mid-Atlantic Ridge are the birthplace of new ocean crust.

A major part of the scientific investigation was seafloor sampling, not only for biological purposes, but also to understand what the seafloor was composed of. The seafloor sediment samples amounted to hundreds of tonnes, and formed the basis for the first map of seafloor deposits. A major step forward in the understanding of the seafloor environment was made by a scientists named John Murray, who realised that the composition of much of the seafloor sediment reflects the type of plankton living in the overlying water column. Among the hundreds of tons of seafloor mud, they also discovered the presence of manganese nodules rich in copper, nickel and cobalt, and tiny particles of magnetic material which proved to be small meteorites.

Sounding measurements taken during the expedition provided the world with the first map showing the true outline of the ocean basins. The deepest measurement they made was in the Marianas trench, now known to be the deepest part of the ocean in the world. Here, scientists measured a depth of 8,200m - a remarkable measurement given the somewhat crude method used! Modern measurements place the maximum depth of the Marianas Trench at nearly 11,000m.

The achievements and discoveries of the Challenger expedition are truly remarkable, and led to many of the greatest steps forward in marine science. But the work didn't finish when the ship came into dock at the end of its journey in Spithead on 24th May 1876. A team of 100 scientists, working under John Murray, continued the research on the samples brought back from sea and the results were published in 'The Report of the Scientific Results of the Exploring Voyage of H.M.S. Challenger during the years 1873-76', which took 11 years to compile and spans 50 volumes! John Murray described the achievments of the voyage as "the greatest advance in the knowledge of our planet since the celebrated discoveries of the fifteenth and sixteenth centuries".

Find out more about ocean exploration:

Ocean exploration: Part 1
Ocean exploration: Part 2
Manned exploration of the deep
The Challenger Expedition
Ocean exploration timeline
Navigation: Dead reckoning
Early navigation
Improved navigation
Modern navigation
Mapping the seafloor
Exploring below the seabed

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February 2007