The Challenger Deep is the deepest known point in the oceans, 10,911 meters (35,797 feet) deep at its maximum, near .

It is in the Pacific Ocean, off the island of Guam in the Mariana Islands group at the southern end of the Mariana Trench.

The Challenger Deep got its name after the survey ship Challenger II, which was surveying the trench in 1951. On 23 January, 1960, the US Navy Bathyscaphe Trieste descended to 10,916 meters (35,813 feet) deep in the trench.

In 1984, a Japanese survey vessel using a narrow, multi-beam echo sounder took a measurement of 10,923 meters (35,838 feet).

A Japanese robotic deep-sea probe, known as Kaiko, broke the depth record for unmanned probes when it reached the bottom of the Challenger Deep on 24 March, 1995. Created by the Japan Marine Science and Technology Center (JAMSTEC), it was one of the rare few unmanned deep-sea probes in operation that could dive deeper than about 6000 meters (19,680 feet). Its recorded depth of 10,911 m (35,797 ft) for the Challenger Deep, is believed to be the most accurate measurement taken yet. Unfortunately, Kaiko was lost at sea on 29 March, 2003, after just more than 8 years of service, when one of the secondary cables snapped during an approaching typhoon. Currently no other operational vehicle exists that is capable of reaching the same depths, and no other manned vehicle has come to the same depth as the Trieste.

Recently, an analysis of the sediment samples collected by Kaiko before she sank, published in Science, Vol 307, Issue 5710, pq. 689[1], announced the discovery of simple organisms at 10,900 meters water depth. While similar lifeforms have been known to exist in shallower ocean trenches (>7,000 m) and on the abyssal plain, the lifeforms discovered in the Challenger Deep possibly represents an independent taxa from these shallower ecosystems.

Out of the 432 organisms collected, the overwhelming majority of the sample consisted of simple, soft-shelled foraminifera, with four of the others representing species of the complex, muti-chambered genera Leptohalysis and Reophax. Overall, 85% of the specimens consisted of organic soft-shelled allogromids. This is unusual compared to samples of sediment-dwelling organisms from other deep-sea environments, where the percentage of organic-walled foraminifera ranges from 5% to 20% of the total. As small organisms with hard calcated shells have trouble growing at extreme (10,000 m) depths because the water at that depth is severely lacking in calcium carbonate, scientists theorize that the preponderance of soft-shelled organisms at the Challenger Deep may have resulted from the typical biosphere present when the Challenger Deep was shallower than it is now. Over the course of six to nine million years, as the Challenger Deep grew to its present depth, many of the species present in the sediment died out or were unable to adapt to the increasing water pressure and changing environment. The remaining species may have been the ancestors of the Challenger Deep's current denizens.