The Challenger was a 2,300-ton British corvette whose scientific staff conducted (1872-76) oceanographic surveys in the Atlantic, Pacific, and Indian oceans in the first attempt to map the ocean floor and describe horizontal and vertical temperature and chemical distribution. Considered to mark the beginning of modern oceanography, the surveys were the basis for all subsequent expeditions. The scientific staff, headed by Sir C. W. Thomson, mapped 364 million sq km (140 million sq mi) of ocean floor and discovered 4,417 new species and 715 new genera. The journey, the first circumnavigation for scientific purposes, covered 69,000 miles. It led to studies of the relation of temperature to pressure. The Challenger performed a sounding of 26,850 ft off the Mariana Islands. The 77 seawater determinations proved that the composition of seawater is relatively constant.