On 21 December 1872, the British naval corvette HMS Challenger sailed from Portsmouth, England on an historic endeavor. Although the sophisticated steam-assisted sailing vessel had been originally constructed as a combat ship, her instruments of war had been recently removed to make room for laboratories, dredging equipment, and measuring apparatuses. She and her crew of 243 sailors and scientists set out on a long, meandering circumnavigation of the globe with orders to catalog the ocean's depth, temperature, salinity, currents, and biology at hundreds of sites–an oceanographic effort far more ambitious than any undertaken before it.

For three and a half long, dreary years the crew spent day after day dredging, measuring, and probing the oceans. Although the data they collected was scientifically indispensable, men were driven to madness by the tedium, and some sixty souls ultimately opted to jump ship rather than take yet another depth measurement or temperature reading. One day in 1875, however, as the crew were "sounding" an area near the Mariana Islands in the western Pacific, the sea swallowed an astonishing 4,575 fathoms (about five miles) of measuring line before the sounding weight reached the floor of the ocean. The bedraggled researchers had discovered an undersea valley which would come to be known as the Challenger Deep. Reaching 6.78 miles at its lowest point, it is now known to be the deepest location on the whole of the Earth. The region is of such immense depth that if Mount Everest were to be set on the sea floor at that location, the mighty mountain's peak would still be under more than a mile of water.