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Throughout human history, the sea has yielded an abundance of resources for man's existence and provided efficient routes for exploration and transportation. In return, it has exacted a toll in terms of human life and property. The fear and respect that it earned from those who ventured out upon its surface was itself a deterrent to learning more about its mysteries. The physical restrictions of penetrating the sea made sub-surface exploration nearly impossible.

The most immediate restriction was air supply. Only the most disciplined divers could stay under for more than a few minutes. As scientists began to recognize the sea as a realm to be explored, this was their first hurdle.

In 1716, the English astronomer, Edmund Halley, whose interests in the universe included the earth, invented a wooden diving bell which was open at the bottom. The significance of Halley's bell was the system developed with it to provide its occupants air. The trapped air in the bell was resupplied by sending down weighted barrels of fresh air. Divers could also venture from the bell with the aid of leather helmets and leather air hoses.

The next major restriction to be surmounted was the tremendous water pressure which increases with depth. While some sea creatures have adapted to this environment, an unprotected human can only dive to about 330 ft (100 m) even with an air supply. The diving suits, bells, and submarines invented in the eighteenth and nineteenth centuries were not constructed well enough to protect divers very far below the surface.

The collaboration in the late 1920s of two scientists, William Beebe, a naturalist from Columbia University in New York, and Otis Barton, an engineer at Harvard University in Boston, led to the invention of the bathysphere, the first ever deep sea exploration vessel.

The sphere was nearly 5 ft (1.5 m) in diameter, had steel walls and weighed 5,400 lb (2,451 kg). It had a circular manhole at the top and three windows made of thick fused quartz. Air was released from two tanks and chemicals were used to absorb moisture and carbon dioxide. It was equipped with a telephone and searchlight. The sphere was tethered to the surface vessel by a steel cable.

After two unmanned test dives, Beebe and Barton made the first manned descent on June 6, 1930, to a world-record depth of 800 ft (244 m). Continued dives led to a record of 3,028 ft (923 m) in the Atlantic Ocean off Bermuda.

The primary purpose of the bathysphere was to explore rather than to set records. The explorations led to discovery of deep sea plant and animal species and observation of already known species. It also gave scientists new knowledge of submarine topography, geology, and geomorphology.

After some improvements to his diving vessel, Barton set a final diving record of 4,501 ft (1,372 m) in the Pacific Ocean off of Southern California in 1949. But by this time efforts led by Swiss physicist Auguste Piccard were being made to develop the successor to the bathysphere, the self-propelled bathyscaphe.

The bathysphere was limited to vertical travel. It was constantly dependent on its umbilical connection to its mothership. Besides visual observation, other exploration activities, such as specimen collection, were difficult with the sphere. Increased water pressures and potential problems with the support line, including the shear weight of the steel cable, made the deeper dives ever more risky.

The development of the bathyscaphe in the early 1950s would allow scientists to overcome the third, and perhaps final, obstacle to deep sea exploration, the need for freedom and independence for meaningful research.

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Science EncyclopediaScience & Philosophy: Ballistic galvanometer to Big–bang theory