The very earliest "explorations" of the sea depended on human endurance, that is, the depth a person could sustain a dive. Our ancient ancestors certainly explored the near shore. The Polynesians dived from their seagoing outrigger canoes, but the depth they could explore was limited to relatively shallow water. The women who dive for pearls in and near Japan and the Greeks who dive for sponges have achieved phenomenal endurance records (presumably in ancient as well as modern times) for holding their breath, but diving for pearl-bearing oysters or for sponges requires perseverance for searching not for depth.
Scientific study of the physics of the deep sea began when the French mathematician, astronomer, and scientist Pierre Simon de Laplace (1749-1827) used only tidal motions along the shores of West Africa and Brazil to calculate the average depth of the Atlantic Ocean. He estimated this average to be 13,000 ft (3,962 m), which scientists later proved with soundings over the ocean to be relatively accurate. Investigations of the sea bottom were begun when submarines were manufactured, and soundings were used to lay submarine cables.
Nineteenth and twentieth century technology has caused an explosion in the exact sciences. The captains of sailing vessels made precise ships' logs in the early nineteenth century that proved valuable in early oceanography. These were compiled by Matthew Fontaine Maury (1806-1873), who set documentation standards later followed by many international congresses on oceanography and other sciences of the sea. The expeditions of Captain James Cook (1728-1779) and the polar explorers (notably Sir James Ross [1800-1862] who explored the North Pole with Sir William Parry [1790-1855] as well as the Antarctic Region and his uncle Sir John Ross [1777-1856] who was also an explorer of the North Pole) added more information about oceanic surfaces.
In the mid-1800s, Norwegian scientists proved life exists in the deep sea when they recovered a stalked crinoid from a depth of 10,200 ft (3,109 m). In 1870, the British began the first expedition strictly to explore the deep ocean. The H.M.S. Challenger expedition left England at Christmastime in 1872 and spent four years conducting oceanographic studies in the oceans of the world, returning to England in May 1876. The ship's crew was under the command of Sir George Nares, and Sir Charles Wyville Thomson (1830-1882) was the chief scientist on board. The crew is credited with discovering 715 new genera and 4,417 new species of marine organisms. At about the same time, the German ship the S.M.S. Gazelle made observations of southern waters including the South Atlantic, South Pacific, and Indian Oceans. The U.S.S. Tuscarora cruised the North Pacific to make soundings for the trans-Pacific cable line and recorded many other scientific observations along with the soundings.
Science EncyclopediaScience & Philosophy: Two-envelope paradox to VenusUnderwater Exploration - History, Oceanography, Instrumentation, Diving Tools And Techniques, Deep-sea Submersible Vessels, Key Findings In Underwater Exploration - Deep-sea pioneers