The Mariana Trench is the deepest oceanic trench on Earth, located in the western Pacific Ocean, about 200 kilometers (124 miles) east of the Mariana Islands. It is crescent-shaped and measures about 2,550 km (1,580 mi) in length and 69 km (43 mi) in width. The maximum known depth is 10,984 ± 25 meters (36,037 ± 82 ft; 6,006 ± 14 fathoms; 6.825 ± 0.016 mi) at the southern end of a small slot-shaped valley in its floor known as the Challenger Deep. The bottom of the Mariana Trench is about 35,876 feet (10,935 meters) deep, making it deeper than Mount Everest is tall.

Measuring the greatest depths in the Mariana Trench is an exceedingly difficult task, given the technical challenges of delivering instrumentation to such a remote location and then obtaining accurate readings. The first attempt was made in 1875 during the Challenger Expedition (1872–76), when a sounding of 26,850 feet (8,184 meters) was obtained near the southern end of the trench. In 1899 Nero Deep (31,693 feet . NOAAs estimate comes from a 2021 study in the journal Deep Sea Research Part I: Oceanographic Research Papers, based on data from a 2020 voyage.