Long, narrow, and mountainous submarine lineation that generally separates ocean-floor ridges differing in depth by as much as 1 mi (1.6 km). The largest fracture zones, in the eastern Pacific, are more than 1,000 mi (1,600 km) long and 60–125 mi (100–200 km) wide. Numerous shorter fracture zones in the Atlantic are associated with the Mid-Atlantic Ridge.

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Accumulation of land-derived sediment on the seafloor; a fan is shaped like the section of a cone, with its apex at the mouth of a subbmarine canyon. The sediments consist largely of sandy material that drops from the canyon current in successively finer layers. Submarine fan valleys, with low relief and natural levees, often occur on submarine fans. Several fans may coalesce laterally.

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Narrow, steep-sided underwater valley cut into a continental slope. Submarine canyons resemble river canyons on land, usually having steep, rocky walls. They are found along most continental slopes. Those of the Grand Bahama Canyon, which are thought to be the deepest, cut nearly 3 mi (5 km) deep into the continental slope. Most submarine canyons extend only about 30 mi (50 km) or less, but a few are more than 200 mi (300 km) long.

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Naval vessel capable of operating underwater for sustained periods. In the 18th and 19th centuries, American inventors such as David Bushnell (1742?–1824) and Robert Fulton experimented with hand-powered submarines. In 1898 John P. Holland (1840–1914) launched the Holland, which had both a gasoline engine for surface locomotion and a battery-powered electric motor for submerged cruising; it was purchased by the U.S. government in 1900. The innovations of Simon Lake (1866–1945) were adopted first in Europe and later in the U.S. By the eve of World War I, all major navies had diesel-electric submarines. German U-boats were an especially potent threat; through World War II they introduced such innovations as the snorkel, which supplied fresh air to the diesel engine without having to surface the boat. Nuclear-powered submarines began service with the launching of the USS Nautilus in 1954. The abundant power provided by uranium-fueled reactors means that nuclear submarines can remain submerged and operate at high speed indefinitely. Only the navies of the U.S., Russia, Britain, France, and China have nuclear-powered submarines; other navies rely on conventional diesel-electric power. Subs may be armed with torpedoes, cruise missiles, or ballistic missiles fitted with nuclear warheads. Because they are so difficult to locate, they are of great importance in the forces of almost all maritime states. Seealso depth charge; sonar; Trident missile.

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