Ocean-floor exploration

The exploration of the ocean floor by probes, submersibles, and remotely operated vehicles is a relatively recent human enterprise. Much of what is known about the floor of the oceans—from its geography, the details of its geology, and its unique biology—has been discovered since 1960.

Early Exploration

Scientists know more about the surface of the moon than about the floor of the earth's oceans. The reason for this surprising lack of knowledge is twofold. First, more than half of the planet lies at depths of more than 3 kilometers, greater than humans can explore at first hand. Second, engineering craft (crewed or uncrewed) that can travel to such great depths under positive control became technologically possible only relatively recently. Since the emergence of such technology, including submersibles and remotely operated devices, scientists are discovering an often alien environment in a vast and unexplored landscape that covers three-fourths of the earth.

The first explorers of the ocean floor charted harbor basins by the topography they could actually see or from soundings taken with weighted lines, which represented only a tiny fraction of the earth's ocean bottom. The first recorded mid-ocean sounding was accomplished by the Spanish explorer Ferdinand Magellan in 1521. He spliced two sounding lines together and lowered them over the side of his ship in the Tuamotu Archipelago until they ran out. Although the 365 meters of line did not reach bottom, Magellan immodestly declared that he had discovered the deepest part of the ocean. The first modern sounding was taken in 1840 in the South Atlantic, measuring a depth of 4,434 meters. The first map of the ocean floor was constructed using approximately 7,000 soundings in 1895. All these soundings were taken with line and weight, which was the only method available at that time to explore the ocean floor.

The first comprehensive ocean-floor soundings were made possible with the advent of acoustic echo sounding in 1920, but the method was not widely used until the 1940s. Considering every recorded sounding prior to World War II, there was, on the average, only one sounding per 2,500 square kilometers of ocean bottom. The use of submarines for warfare became widespread during the 1940s. They required the ability to determine the exact ocean-floor topography. With the subsequent vast improvement of echo sounding and sonar technologies, a new age of exploration was engendered. By the early 1970s, a combination of satellite navigation (which enabled accurate geographical positioning of soundings) and precise echo sounders enabled a rapid charting of the ocean floor, a branch of the science called cartography. Sunlight does not penetrate to depths greater than about 100 meters under the very best of circumstances. Hence, for scientists to chart the geography of the sea floor, they must combine individual soundings to form seafloor charts, ultimately covering vast areas.

A picture has developed of the sea floor that has revealed a widely variable seascape, at least as mutable as the continents but quite unlike above-water, continent-born landscapes. The underwater topography is unaffected by the powerful erosional forces found on the surface; the submarine world is shaped by forces that are unique to the oceans. The major submarine features (called the geomorphic domain) are deep-sea trenches, rifts (deep valleys with steep sides), flat-topped undersea volcanic cones, and fracture zones of very long linear cracks and fissures.

Oceanographic research ships have prowled the oceans for the past two centuries. Such ships as Challenger, Discovery, and Endeavor used lines and weights to determine depths. With a slight modification, they used the same lines and weights to carry sampling devices to the ocean floor to obtain specimens for later examination on the surface. Such remote-sensing devices were called coring devices, and some were complex enough to carry several types of sampling equipment, from water to temperature probes and even sea-life traps of various kinds. Nevertheless, all these sampling devices were not usually a part of a single probe and, more often than not, were sent down independently.

Crewed Submarines and ROVs

Humans have also studied the ocean floor personally, first as free divers, then using the breathing apparatus invented in the 1940s. Oxygen and nitrogen are toxic and dangerous even at 100 meters, however, thus limiting a diver with a self-contained breathing apparatus to above this level. Thus, it became necessary to invent manned submarines that could dive deeper. In deep-sea exploration, submarines are a form of remote-sensing device in that they shield the people within them from the tremendous ocean pressures by thick hulls.

Such a submarine is the Alvin, which is capable of dives as deep as 6,100 meters. Like most submersibles of this type, Alvin has a robot hand that extends from the body to enable the operators inside to perform work outside the submarine. This famous submersible has also allowed investigators to observe life-forms around submerged volcanic vents on the ocean floor. One of Alvin's most famous dives was made in 1986 to investigate the sunken hull of the Titanic in 4,000 meters of water in the North Atlantic. It was on this dive, however, that the limitations of such a manned submersible became apparent. The trip from the surface to the ocean floor alone required more than two hours and the same time to return. That limited the amount of time Alvin had to work on the ocean floor to merely a few hours.

Oceanographers and other scientists were thus led to invent devices called remotely operated vehicles (ROVs). These devices perform work on the ocean floor with as much dexterity as a human inside a submersible. ROVs carry cameras with capabilities from still photographs to live television. Some maneuver independently on tethers and have remotely operated arms. One inventor of such devices, and the discoverer of the Titanic's final resting place, is Robert Ballard of Woods Hole Oceanographic Institution in Massachusetts. Ballard, with funding from the U.S. Navy and the National Geographic Society, invented three different types of remote-sensing ROVs for exploring the ocean floor. Ballard's ROVs include the Argo, a camera sled equipped with still and live cameras mounted on a platform towed behind a surface ship. Others are the Jason and Jason Jr. Jason Jr. rides to the ocean floor on Jason, then detaches and is capable of executing a wide range of movements. According to Ballard, such devices free humans from the inherent dangers of piloting crewed submersibles to great depths, eliminate the excessive travel time from and to the surface, obviate the need for a complex manned vehicle, and give surface operators a "telepresence" on the ocean floor.

Even with the advances in ROVs, many scientists (and thrill seekers) continue to value the experience and knowledge to be gained by manned expeditions to the depths of the ocean floor. In particular, the challenge of reaching the deepest known points of the ocean has long attracted both adventurers and researchers. In 1960, Don Walsh and Jacques Piccard used the vessel Trieste, a bathyscaphe, to descend into the fittingly named Challenger Deep of the Mariana Trench, approximately 200 miles of the coast of Guam. It would be over fifty years before anyone would return there, the deepest known place in the world at nearly 36,000 feet below sea level. Then, in 2012, filmmaker and explorer James Cameron made the journey in the custom-made submersible Deepsea Challenger. Compared to the earlier Mariana voyage, Cameron's expedition was able to record footage and take samples of the virtually unknown environment it encountered. In 2018, Victor Vescovo piloted a historic 8,376 meter (nearly five miles) dive to the deepest part of the Atlantic Ocean, the Puerto Rico trench. However, as part of a five ocean diving mission, this dive was more geared toward exploration than research. The following year, Vescovo, accompanied by Dawn Wright, set records for the deepest dives ever recorded in their vessel, Limiting Factor, in the Indian Ocean, the South Pacific, and the Arctic Ocean. In 2020, Vescovo piloted a vessel that reached depths of more than eleven thousand meters and was accompanied by oceanographer Dr. Kathryn Sullivan and explorer Vanessa O'Brien, the first women to reach the bottom of Challenger Deep.

Ocean-Floor Characteristics

Undersea remote-sensing devices have revealed that the ocean floors are complex, distinctive, active landscapes with vast mountain ranges, plains, highlands, valleys, and active volcanic vents. Far from being a static, quiescent place, the ocean floor is a dynamic, constantly active geologic area of considerable interest to geologists.

The earth's continents drift across the planet in a slow, never-ending flux measured in billions of years; vital information describing this process may be found at the fracture zones that mark the boundaries between the earth's continental plates. Most of the earth's earthquakes are centered on such fracture zones. In addition, most of the planet's active volcanoes lie on the ocean floor. Only by extensive on-site study of these regions can scientists hope to fully understand what they seek to know about the planet's earthquakes and volcanoes. Because the very character of the earth's crust is different at these continental boundaries (as opposed to the upper, higher regions of crust on the continents' surface levels), scientists need to study the nature of the active regions far below sea level.

At the fracture zones, magma from deep inside the earth wells up to near the surface, heating the cold seawater to very high temperatures (hot enough, in some cases, to melt lead) and spewing forth mineral-rich deposits from hydrothermal vents into the water. At these places, strange life-forms have been discovered that do not rely on sunlight (photosynthesis) for their survival. They are entirely dependent on the minerals from the vents, surviving in a metabolic state called chemosynthesis. One of these deep-sea animals, called a tube worm, was discovered by Ballard in 1977 onboard the Alvin at a depth of 6,100 meters near hydrothermal vents off the Galápagos Islands.

Such astonishing discoveries give rise to questions that challenge fundamental biological assumptions. For example, did the life-forms evolve independently from surface, photosynthetically supported plants, and if so, what does that portend for the possibility of chemosynthetically evolved life throughout the universe? If such development is possible, then there may be vast reservoirs of life in the universe that have evolved in the absence of a neighbor star, long considered the most basic requirement for the development of life-forms.

Exploration of the ocean floor has applications for the economies and energy needs of modern civilization. On the vast ocean-floor regions lie metal ores (such as manganese nodules) that may one day supply a significant percentage of industrial requirements for this resource. Also, a blanket of very cold water (near or slightly below freezing) lies on much of the ocean floor, which may one day be exploited for its energy transfer capacity in ocean thermal energy power plants.

Significance

As spectacular as is the emergent image of the ocean floor derived from the millions of soundings taken thus far, it remains merely a coarse outline of the total picture of the underwater domain. As remote-sensing devices improve, five fundamental goals in addition to basic soundings will be accomplished: a more complete understanding of the extent and details of available resources on the ocean floor; an expansion of humankind's telepresence on the deep frontiers of the ocean floor effected from remote locations; continued exploitation of the available resources on the ocean floor, such as petroleum, mineral, and food resources; collection and categorization of a vast amount of basic scientific information on the geology, available energy, and biology of the sea floor; and ongoing exploration to discover and investigate life-forms previously unknown or entirely unsuspected.

These discoveries will both enrich science's base of knowledge about the earth and make the ocean's resources available to the world's peoples. The technology developed to explore the ocean floor will also be employed in other scientific endeavors. For example, such remote-sensing capabilities will probably be adapted for use in space exploration.

Principal Terms

acoustic echo sounding: a method of determining the depth of the ocean floor that measures the time of a reflected sound wave and relates that to distance

cartography: the science of mapmaking; maps or charts of the ocean floor are developed by linking the individual points of ocean-floor depths

chemosynthesis: the synthesis of organic substances by living organisms through the energy of chemical reactions

coring devices: devices that drill into ocean-floor sediments to provide scientists with information on the composition of the seabed

fracture zones: areas that define the edges between the continents on the sea floor

geomorphic domain: major underwater features that define the appearance of a seafloor area

hydrothermal vents: areas where very hot water is expelled from volcanically active vents on the sea floor

remotely operated vehicle (ROV): a submersible operated from a remote location; for example, a robot that explores the sea floor while operated by tether from a surface ship

sounding: the measurement of depth; a sounding line is a line used for the measurement of depth

telepresence: the ability of a human to explore an area remotely by live television

Bibliography

Ballard, Robert D., and Malcolm McConnell. Adventures in Ocean Exploration: From the Discovery of the Titanic to the Search for Noah’s Flood. Warner Books, 2001.

Bar-Cohen, Yoseph, and Kris Zacny, editors. Drilling in Extreme Environments: Penetration and Sampling on Earth and Other Planets. Wiley-VCH, 2009.

Clash, Jim. "Voyage to the Bottom of the Earth." Forbes, 18 June 2020, www.forbes.com/sites/jimclash/2020/06/18/voyage-to-the-bottom-of-the-earth/#309a0e592229. Accessed 13 Nov. 2024.

“Deepsea Challenge: The Expedition.” National Geographic, 2017, www.deepseachallenge.com/the-expedition/. Accessed 13 Nov. 2024.

Dolgoff, Anatole. Physical Geology. Houghton Mifflin Harcourt, 1999.

Exon, Neville. “Scientific Drilling beneath the Oceans Solves Earthly Problems.” Australian Journal of Maritime and Ocean Affairs, vol. 2, no. 2, 2010, pp. 37–47. doi.org/10.1080/18366503.2010.10815654. Accessed 13 Nov. 2024.

Garrison, Tom S. Oceanography: An Invitation to Marine Science. 10th ed., Cengage Learning, 2022.

Haymon, Rachel M., et al. “Manifestations of Hydrothermal Discharge from Young Abyssal Hills on the Fast-Spreading East Pacific Rise Flank.” Geology, vol. 33, no. 2, 2005, pp. 153–156. doi.org/10.1130/G21058.1. Accessed 13 Nov. 2024.

"How Much of the Ocean Has Been Explored?" NOAA Ocean Exploration, National Oceanic and Atmospheric Administration, oceanexplorer.noaa.gov/facts/explored.html. Accessed 13 Nov. 2024.

Kennish, Michael J. Practical Handbook of Marine Science. 4th ed., CRC Press, 2022.

Koblick, Ian G., and James W. Miller. Living and Working in the Sea. 2nd ed., Best Publishing, 1995.

Prothero, D. R., and Robert H. Dott. Evolution of the Earth. 8th ed., McGraw-Hill, 2009.

Seibold, Eugen, and Wolfgang H. Berger. The Sea Floor: An Introduction to Marine Geology. 4th ed., Springer, 2018.

Segar, Douglas A., and Elaine Stamman Segar. An Introduction to Ocean Sciences. 4th ed., W. W. Norton & Company, 2018.

Singh, Satish C., et al. “Discovery of a Magma Chamber and Faults Beneath a Mid-Atlantic Ridge Hydrothermal Field.” Nature, vol. 442, 2006, pp. 1029–1032. doi.org/10.1038/nature05105. Accessed 13 Nov. 2024.

Weiner, Jonathan. Planet Earth. Bantam Books, 1986.