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The History of Deep Sea Exploration

The depths of the sea have been investigated with precision only during comparatively recent years; compared to the other areas of geological research, they still form a relatively unexplored domain.

Modern scientific study of the deep sea can be said to have begun when the French scientist Pierre Simon de Laplace calculated the average depth of the Atlantic Ocean from tidal motions registered on the Brazilian and African coasts. He determined this depth to be 3962 m (13,000 ft), a value proven quite accurate by later soundings. The first investigations of the sea bottom were undertaken because of the need for accurate soundings when laying submarine cables. That life exists at great depths was discovered in 1864, when Norwegian researchers sampled a stalked crinoid at a depth of 3109 m (10,200 ft). The most important knowledge of deep-sea conditions has been gained since 1870, beginning with the Challenger expedition sent out by the British government in 1872. It engaged in global oceanographic investigations for nearly four years, during which 715 new genera and 4417 new species of marine organisms were discovered.

Oceanographic Instrumentation

The first instruments used for the investigation of the sea bottom was the sounding weight, with which British explorer Sir James Clark Ross (1800-62) reached a depth of 3700 m (12,140 ft) in 1840.

The sounding weights used on the Challenger, called Baillie sounding machines, were provided with a tube into which a sample of the seabed was forced when the weight hit the bottom of the ocean.

Also used on the Challenger were dredges and scoops, suspended on ropes, with which samples of the sediment and biological specimens of the seabed could be obtained.

A modern version of the Baillie sounding machine is the gravity corer. The corer consists of an open-ended tube with a lead weight and a trigger mechanism that releases the corer from its suspension cable when the corer is lowered over the seabed and a small weight touches the ground. The corer falls into the seabed and penetrates it to a depth of up to 10 m (33 ft). By lifting the corer, a long, cylindrical sample is extracted in which the structure of the seabed's layers of sediment is preserved. Samples of deeper layers can be obtained with a corer mounted in a drill. The drilling vessel JOIDES Resolution is equipped to extract cores from depths of as much as 1500 m (4900 ft) below the ocean bottom.

Since World War II, echo-sounding techniques have been widely used to measure the depth of the sea bottom. Acoustic pulses are transmitted from the ship; the time registered for the reflection of the sound wave is a measure of the water's depth. By registering the time lapses between outgoing and returning signals continuously on paper tape, a continuous mapping of the seabed is obtained. Much of the ocean floor has been mapped in this way.

Other instruments for deep-sea exploration are high-resolution television cameras, movie cameras, thermometers, pressure meters, flow meters, and seismographs. These instruments are either lowered to the sea bottom on long cables or attached to submersible buoys; they sometimes are provided with a sound source, making depth determination possible. Deep-sea currents can be determined by floats carrying an ultrasonic sound source so that their movements can be followed aboard the research vessel. Such vessels themselves require precise navigational instrumentation, such as satellite navigation devices, and special positioning systems that keep the vessel in a fixed position relative to a sonar beacon on the bottom of the ocean.

Oceanographic Submersibles

The American explorer Charles William Beebe was the first to observe marine species at depths that could not be reached by a diver. He and the engineer Otis Barton (1899- ) designed a spherical steel vessel called a BATHYSPHERE, (q.v.) that could be lowered from a ship, suspended from a cable. In 1930 Beebe and Barton reached a depth of 435 m (about 1425 ft), and in 1934 a depth of 923 m (3028 ft). The danger of this submersible was that if the cable broke, the occupants could not return to the surface. With this in mind, Swiss physicist Auguste Piccard designed his first BATHYSCAPHE, (q.v.), a navigable deep-sea vessel consisting of a pressure sphere that is kept buoyant by a float (a large container filled with gasoline). With this bathyscaphe, Piccard reached (1954) a depth of 4000 m (13,125 ft). In 1960 his son Jacques Piccard (1922- ) reached the record depth of 10,916 m (about 35,800 ft) in the Mariana Trench off the island of Guam, with Trieste (the second bathyscaphe built by Piccard in 1953).

A large number of occupied submersibles for deep-sea exploration are now employed by different countries around the world. Among them is the American-built Alvin, which can dive to about 3600 m (about 12,000 ft), and which is equipped with underwater lights, cameras, a television system, and a mechanical manipulator to collect bottom samples. Unmanned, robot submersibles are also being used for underwater exploration. Capable of descending to depths of as much as 6000 m (20,000 ft), one such craft, called Argo, was used in 1985 to locate the wreck of the Titanic, and a smaller robot, called Jason, was used to explore the wreck.

Scientific Results

The first large exploration using occupied submersibles was the French-American Mid-Ocean Undersea Study (FAMOUS) project. In 1974 the Alvin (operated by the Woods Hole Oceanographic Institution), the French bathyscaphe Archimède, and the French diving saucer Cyane, assisted by support ships and the Glomar Challenger, explored the great Rift Valley of the Mid-Atlantic Ridge, southwest of the Azores. The Rift Valley is considered by geologists as the separation between the Eurasian plate and the North American plate of the earth's crust, and it constitutes one of the many sites in the ocean bottom where molten rock oozes forth to form new crust. About 5200 photographs of the region were taken, and samples of relatively young solidified magma were found on each side of the central fissure of the Rift Valley, giving additional proof that the seafloor spreads at this site at a rate of about 2.5 cm (about 1 in) per year. In a series of dives (1979-80) into the Galápagos rift, off the coast of Ecuador, French, Mexican, and U.S. scientists found chimneylike vents, nearly 9 m (nearly 30 ft) high and about 3.7 m (about 12 ft) across, discharging a mixture of hot water (up to 300° C/570° F) and dissolved metals in dark, smokelike plumes. These hot springs play an important role in the formation of deposits that are enriched in copper, nickel, cadmium, chromium, and uranium.

An article from Funk & Wagnalls® New Encyclopedia. © 2005 World Almanac Education Group. A WRC Media Company. All rights reserved. Except as otherwise permitted by written agreement, uses of the work inconsistent with U.S. and applicable foreign copyright and related laws are prohibited.