Scientists and explorers have been searching to determine the exact location and depth of the deepest part of the ocean since the HMS Challenger made the first sounding of the Mariana Trench in 1875. The consensus is that the deepest abyss in the ocean is in the Challenger Deep, a portion of the Mariana Trench with depths greater than 10 000 m. Since the HMS Challenger II returned to the Mariana Trench in 1952, 14 estimates of the deepest depth of the ocean have been made. Estimates of the location and maximum depth are as diverse as the methods used including wire soundings, explosives, single and multibeam sonars, and remotely operated and manned submersibles. During an Office of Naval Research supported expedition to the Challenger Deep in 2014, two free falling passive acoustic instruments were deployed. The implosion of one instrument was recorded by the other when both were at depths greater than 8000 m. Multiple reflections from the seafloor and sea surface of the sound generated by the implosion were used to determine the depth of the Challenger Deep. The result was the most constrained estimate of the deepest part of the ocean, 10 991 ± 6 m.