Deep-sea exploration is the investigation of physical, chemical, and biological conditions in the ocean waters and sea bed beyond the continental shelf, for scientific or commercial purposes. Deep-sea exploration is an aspect of underwater exploration and is considered a relatively recent human activity compared to the other areas of geophysical research, as the deeper depths of the sea have been investigated only during comparatively recent years. The ocean depths still remain a largely unexplored part of the Earth, and form a relatively undiscovered domain.
Scientific deep-sea exploration can be said to have begun when French scientist Pierre-Simon Laplace investigated the average depth of the Atlantic Ocean by observing tidal motions registered on Brazilian and African coasts circa the late 18th or early 19th century. However, the exact date of his investigation is unknown. He calculated the depth to be , a value later proven quite accurate by echo-sounding measurement techniques.Deep Sea Exploration." World of Earth Science. Ed. K. Lee Lerner and Brenda Wilmoth Lerner. Gale Cengage, 2003. eNotes.com. 2006. 7 Dec, 2009 <http://www.enotes.com/earth-science/deep-sea-exploration > Later on, due to increasing demand for the installment of submarine cables, accurate measurements of the sea floor depth were required and the first investigations of the sea bottom were undertaken. The first deep-sea life forms were discovered in 1864 when Norwegian researchers Michael Sars and Georg Ossian Sars obtained a sample of a stalked crinoid at a depth of .
From 1872 to 1876, a landmark ocean study was carried out by British scientists aboard HMS Challenger, a screw corvette that was converted into a survey ship in 1872. The Challenger expedition covered , and shipboard scientists collected hundreds of samples and hydrographic measurements, discovering more than 4,700 new species of marine life, including deep-sea organisms. They are also credited with providing the first real view of major seafloor features such as the deep ocean basins.
The first instrument used for deep-sea investigation was the sounding weight, used by British explorer Sir James Clark Ross. With this instrument, he reached a depth of in 1840. The Challenger expedition used similar instruments called Baillie sounding machines to extract samples from the sea bed.
In the 20th century, deep-sea exploration advanced considerably through a series of technological inventions, ranging from the sonar system, which can detect the presence of objects underwater through the use of sound, to manned deep-diving submersibles. In 1960, Jacques Piccard and United States Navy Lieutenant Donald Walsh descended in the bathyscaphe into the deepest part of the world's oceans, the Mariana Trench. On 25 March 2012, filmmaker James Cameron descended into the Mariana Trench in , and, for the first time, filmed and sampled the bottom.
Despite these advances in deep-sea exploration, the voyage to the ocean bottom is still a challenging experience. Scientists are working to find ways to study this extreme environment from the shipboard. With more sophisticated use of fiber optics, satellites, and remote-control robots, scientists hope to, one day, explore the deep sea from a computer screen on the deck rather than out of a porthole.
Milestones
The extreme conditions in the deep sea require elaborate methods and technologies to endure, which has been the main reason why its exploration has had a comparatively short history. Some important milestones of deep sea exploration are listed below:
1521: Ferdinand Magellan tried to measure the depth of the Pacific Ocean with a weighted line, but did not find the bottom. 1810–1827: Antoine Risso, an apothecary from Nice (then part of the Duchy of Savoy) published a series of papers that remained long ignored, describing and naming dozens of fish and crustaceans species collected by fishermen at depths between in the Gulf of Genoa. 1818: The British researcher Sir John Ross independently discovered that the deep sea is inhabited by life when catching jellyfish and worms in about depth with a special device. 1843: Edward Forbes claimed that diversity of life in the deep sea is little and decreases with increasing depth. He stated that there could be no life in waters deeper than , the so-called Abyssus theory. 1850: Near Lofoten, Michael Sars found a rich variety of deep sea fauna in a depth of , thereby refuting the Abyssus Theory.Ludwig Darmstaedter (Hrsg.): Handbuch zur Geschichte der Naturwissenschaften und der Technik, Springer, Berlin 1908, S. 521 1872–1876: The first systematic deep sea exploration was conducted by the Challenger expedition on board the ship led by Charles Wyville Thomson. This expedition revealed that the deep sea harbours a diverse, specialized biota. 1890–1898: First Austrian-Hungarian deep sea expedition on board the ship led by Franz Steindachner in the eastern Mediterranean and the Red Sea. 1898–1899: First German deep sea expedition on board the ship Valdivia led by Carl Chun; found many new species from depths greater than in the southern Atlantic Ocean. 1930: William Beebe and Otis Barton were the first humans to reach the deep sea when diving in the Bathysphere, a spherical steel pressure resistant chamber. They reached a depth of , where they observed jellyfish and shrimp. 1934: The Bathysphere reached a depth of . 1947–1948: Swedish oceanographer Hans Petterson organised and led the Albatross expedition, which went around the equator in 15 months. With a help of a Kullenberg piston corer the expedition for the first time brought up sediment cores from the seafloor and performed deep-sea trawling at below the ocean surface. 1948: Otis Barton set a new record, diving to a depth of in the bathysphere. 1960: Jacques Piccard and Don Walsh reached the bottom of the Challenger Deep in the Mariana Trench, descending to a depth of in their deep sea vessel , where they observed fish and other deep sea organisms. 2012: The vessel , piloted by James Cameron, completed the second crewed voyage and first solo mission to the bottom of the Challenger Deep. 2018: , piloted by Victor Vescovo, completed the first mission to the deepest point of the Atlantic Ocean, diving below the ocean surface to the base of the Puerto Rico Trench. 2019: DSV Limiting Factor made dives to the bottom of the South Sandwich Trench, deepest part of the Southern Ocean, the Sunda Trench in the Indian Ocean, the Horizon Deep in the Tonga Trench in the South Pacific, several dives in the Marianas Trench at Challenger Deep and Sirena Deep and in the Molloy Deep in the Arctic Ocean 2020: As mission specialists on board the vessel Limiting Factor Dr. Kathryn Sullivan and Vanessa O'Brien completed missions piloted by Vescovo, becoming the first women to reach the bottom of Challenger Deep at .
Oceanographic instrumentation
The sounding weight, one of the first instruments used for the sea bottom investigation, was designed as a tube on the base which forced the seabed in when it hit the bottom of the ocean. British explorer Sir James Clark Ross fully employed this instrument to reach a depth of in 1840.Deep-Sea Exploration: Earth's Final Frontier Only a Portion of the Potential of the Oceans Has Been Tapped, but It Is Clear That Exploring and Improving Our Understanding of the Ocean and Its Influence on Global Events Are among Our Most Important Challenges Today Journal article by Stephen L. Baird; The Technology Teacher, Vol. 65, 2005.
Sounding weights used on were the slightly more advanced "Baillie sounding machine". The British researchers used wire-line soundings to investigate sea depths and collected hundreds of biological samples from all oceans except the Arctic. Also used on HMS Challenger were dredges and scoops, suspended on ropes, with which samples of the sediment and biological specimens of the seabed could be obtained.
A more advanced version of the sounding weight is the gravity corer. The gravity corer allows researchers to sample and study sediment layers at the bottom of oceans. 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 . By lifting the corer, a long, cylindrical sample is extracted in which the structure of the seabed’s layers of sediment is preserved. Recovering sediment cores allows scientists to see the presence or absence of specific fossils in the mud that may indicate climate patterns at times in the past, such as during the ice ages. 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 below the ocean bottom. (See Ocean Drilling Program)
Echo-sounding instruments have also been widely used to determine the depth of the sea bottom since World War II. This instrument is used primarily for determining the depth of water by means of an acoustic echo. A pulse of sound sent from the ship is reflected from the sea bottom back to the ship, the interval of time between transmission and reception being proportional to the depth of the water. By registering the time lapses between outgoing and returning signals continuously on paper tape, a continuous mapping of the seabed is obtained. The majority of the ocean floor has been mapped in this way.
High-resolution video cameras, thermometers, pressure meters, and seismographs are other instruments useful for deep-sea exploration. These instruments are either lowered to the sea bottom by cables or attached to submersible buoys. Deep-sea currents can be studied by floats carrying an ultrasonic sound device so that their movements can be tracked from aboard the research vessel. These vessels are equipped with precise navigational instruments, such as satellite navigation and dynamic positioning systems that keep the vessel in a fixed position relative to a sonar beacon on the bottom of the ocean. Magnetometers were used for the first time at the Wreck of the Titanic during a 15 July 2024 expedition, in order to provide metal detection as well as recover on-site artefacts, which is a means often utilised by explorers in examining shipwrecks at such depths given their material accuracy in recognising ferromagnetic material, and are therefore often in high demand by expedition firms.
Oceanographic submersibles
Because of the high pressure, the depth to which a diver can descend without special equipment is limited. The deepest recorded descent made by a freediver is as of 2012. The scuba record is as of June 2005, and 534 metres (1,752 ft) on surface supply on the Comex Hydra 8 experimental dives in 1988.
Atmospheric diving suits isolate the diver from the ambient pressure, and allow divers to reach depths to approximately . Some atmospheric suits feature thrusters that can propel the diver through the water.
To explore greater depths, deep-sea explorers must rely on specially constructed pressure resistant chambers to protect them, or explore remotely. The American explorer William Beebe, also a naturalist from Columbia University in New York, working with fellow engineer Otis Barton of Harvard University, designed the first practical bathysphere to observe marine species at depths that could not be reached by a diver. In 1930 Beebe and Barton reached a depth of , and in 1934. The potential danger was that if the cable broke, the occupants could not return to the surface. During the dive, Beebe peered out of a porthole and reported his observations by telephone to Barton who was on the surface.