In March of 1875, HMS Challenger entered the leg of its journey between the Admiralty Islands and Yokohama, Japan. Attempting to reach Guam, the ship was put off course by "baffling winds" that put the vessel somewhat west of its intended course. Their new path put Challenger over a vast undersea canyon; on 23 March the depth was sounded as 26,850 feet at 11°24′N 143°16′E, the deepest water encountered over the entire expedition. The scientifics on board called the place "Nares Deep", after Challenger's captain, George Nares, though later oceanographers would rename it to honor the entire vessel. It has proven to be the lowest point on Earth, reaching depths of 35,000 feet.

Challenger Deep is part of a much larger series of undersea canyons known as the Mariana Trench. These extraordinarily deep areas are formed at the meeting of two tectonic plates, with one slipping under the other in a process known as subduction. The Trench is the direct opposite of the Mid-Atlantic Ridge, where the tectonic plates are moving apart and allowing magma to erupt into the sea.