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In the late 1960s, deep ocean science was transformed by Alvin, the first DSV (deep submergence vessel). Alvin was named for the scientist who devised and pushed it: Allyn Vine, of the Woods Hole Oceanographic Institution, in Massachusetts.
Alvin was launched in 1964. Capable of operating at depths of 4000m, Alvin has cameras, remotely operated arms, and room for three scientists aboard. It is deployed from the support vessel Atlantis. In 1977 and 1979, Alvin visited and documented the first hydrothermal vents, discovering life at great depth. Still going strong after more than 5000 dives, Alvin famously photographed the sunken Titanic in 1986, and helped to characterise damage to the seafloor after the Deep Water Horizon oil disaster in 2014.
By the way, a league is about 4.8km, and the maximum depth of the sea, in the Marianas Trench, is a bit more than two of them, at 10.9km.