Deepest known point of Earth's seabed
The Challenger Deep is the deepest known point of the seabed of Earth, located in the western Pacific Ocean at the southern end of the Mariana Trench, in the ocean territory of the Federated States of Micronesia.
The GEBCO Gazetteer of Undersea Feature Names indicates that the feature is situated at and has a maximum depth of below sea level. A subsequent study revised the value to at a 95% confidence interval. However, both the precise geographic location and depth remain ambiguous, with contemporary measurements ranging from .
The depression is named after the British Royal Navy survey ships , whose expedition of 1872–1876 first located it, and HMS "Challenger II", whose expedition of 1950–1952 established its record-setting depth. The first descent by any vehicle was conducted by the United States Navy using the bathyscaphe "Trieste" in January 1960. As of July 2022[ [update]], there were 27 people who have descended to the Challenger Deep.
Topography.
The Challenger Deep is a relatively small slot-shaped depression in the bottom of a considerably larger crescent-shaped oceanic trench, which itself is an unusually deep feature in the ocean floor. The Challenger Deep consists of three basins, each long, wide, and over in depth, oriented in echelon from west to east, separated by mounds between the basins higher. The three basins feature extends about west to east if measured at the isobath. Both the western and eastern basins have recorded depths (by sonar bathymetry) in excess of , while the center basin is slightly less deep. The closest land to the Challenger Deep is Fais Island (one of the outer islands of Yap), southwest, and Guam, to the northeast.
Detailed sonar mapping of the western, center and eastern basins in June 2020 by the DSSV "Pressure Drop" combined with manned descents revealed that they undulate with slopes and piles of rocks above a bed of pelagic ooze. This conforms with the description of Challenger Deep as consisting of an elongated seabed section with distinct sub-basins or sediment-filled pools.
Surveys and bathymetry.
Over many years, the search for, and investigation of, the location of the maximum depth of the world's oceans has involved many different vessels, and continues into the twenty-first century.
The accuracy of determining geographical location, and the beamwidth of (multibeam) echosounder systems, limits the horizontal and vertical bathymetric sensor resolution that hydrographers can obtain from onsite data. This is especially important when sounding in deep water, as the resulting footprint of an acoustic pulse gets large once it reaches a distant sea floor. Further, sonar operation is affected by variations in sound speed, particularly in the vertical plane. The speed is determined by the water's bulk modulus, mass, and density. The bulk modulus is affected by temperature, pressure, and dissolved impurities (usually salinity).
1875 – HMS "Challenger".
In 1875, during her transit from the Admiralty Islands in the Bismarck Archipelago to Yokohama in Japan, the three-masted sailing corvette HMS ""Challenger" "attempted to make landfall at Spanish Marianas (now Guam), but was set to the west by "baffling winds" preventing her crew from "visiting either the Carolines or the Ladrones." Their altered path took them over the undersea canyon which later became known as the Challenger Deep. Depth soundings were taken by Baillie-weighted marked rope, and geographical locations were determined by celestial navigation (to an estimated accuracy of two nautical miles). One of their samples was taken within fifteen miles of the deepest spot in all of Earth's oceans. On 23 March 1875, at sample station number #225, HMS "Challenger" recorded the bottom at deep, (the deepest sounding of her three-plus-year eastward circumnavigation of the Earth) at – and confirmed it with a second sounding at the same location. The serendipitous discovery of Earth's deepest depression by history's first major scientific expedition devoted entirely to the emerging science of oceanography, was incredibly good fortune, and especially notable when compared to the Earth's third deepest site (the Sirena Deep only 150 nautical miles east of the Challenger Deep), which would remain undiscovered for another 122 years.
1951 – SV HMS "Challenger II".
Seventy-five years later, the 1,140-ton British survey vessel HMS "Challenger II", on her three-year westward circumnavigation of Earth, investigated the extreme depths southwest of Guam reported in 1875 by her predecessor, HMS "Challenger". On her southbound track from Japan to New Zealand (May–July 1951), "Challenger II" conducted a survey of the Marianas Trench between Guam and Ulithi atoll, using seismic-sized bomb-soundings and recorded a maximum depth of . The depth was beyond "Challenger II"'s echo sounder capability to verify, so they resorted to using a taut wire with "140 lbs of scrap iron", and documented a depth of . The Senior Scientist aboard "Challenger II", Thomas Gaskell, recalled: [I]t took from ten past five in the evening until twenty to seven, that is an hour and a half, for the iron weight to fall to the sea-bottom. It was almost dark by the time the weight struck, but great excitement greeted the reading...In New Zealand, the "Challenger II" team gained the assistance of the Royal New Zealand Dockyard, "who managed to boost the echo sounder to record at the greatest depths". They returned to the "Marianas Deep" (sic) in October 1951. Using their newly improved echo sounder, they ran survey lines at right angles to the axis of the trench and discovered "a considerable area of a depth greater than " – later identified as the Challenger Deep's "western" basin. The greatest depth recorded was , at . Navigational accuracy of several hundred meters was attained by celestial navigation and LORAN-A. As Gaskell explained, the measurement was not more than 50 miles from the spot where the nineteenth-century "Challenger" found her deepest depth [...] and it may be thought fitting that a ship with the name "Challenger" should put the seal on the work of that great pioneering expedition of oceanography.The term "Challenger Deep" came into use after this 1951–52 "Challenger" circumnavigation, and commemorates both British ships of that name involved with the discovery of the deepest basin of the world's oceans.
1957–1958 – RV "Vityaz".
In August 1957, the Soviet 3,248-ton Vernadsky Institute of Geochemistry research vessel recorded a maximum depth of at in the western basin of the Challenger Deep during a brief transit of the area on Cruise #25. She returned in 1958, Cruise #27, to conduct a detailed single beam bathymetry survey involving over a dozen transects of the Deep, with an extensive examination of the western basin and a quick peek into the eastern basin. Fisher records a total of three "Vityaz" sounding locations on Fig.2 "Trenches" (1963), one within yards of the 142°11.5' E location, and a third at , all with depth. The depths were considered statistical outliers, and a depth greater than 11,000 m has never been proven. Taira reports that if "Vityaz"'s depth was corrected with the same methodology used by the Japanese RV "Hakuho Maru" expedition of December 1992, it would be presented as , as opposed to modern depths from multibeam echosounder systems greater than with the NOAA accepted maximum of in the western basin.
1959 – RV "Stranger".
The first definitive verification of both the depth and location of the Challenger Deep (western basin) was determined by Dr. R. L. Fisher from the Scripps Institution of Oceanography, aboard the 325-ton research vessel "Stranger". Using explosive soundings, they recorded at/near in July 1959. "Stranger" used celestial and LORAN-C for navigation. LORAN-C navigation provided geographical accuracy of or better. According to another source RV "Stranger" using bomb-sounding surveyed a maximum depth of at . Discrepancies between the geographical location (lat/long) of "Stranger"'s deepest depths and those from earlier expeditions ("Challenger II" 1951; "Vityaz" 1957 and 1958) "are probably due to uncertainties in fixing the ships' positions". "Stranger"'s north-south zig-zag survey passed well to the east of the eastern basin southbound, and well to the west of the eastern basin northbound, thus failed to discover the eastern basin of the Challenger Deep. The maximum depth measured near longitude 142°30'E was , about 10 km west of the eastern basin's deepest point. This was an important gap in information, as the eastern basin was later reported as deeper than the other two basins.
"Stranger" crossed the center basin twice, measuring a maximum depth of in the vicinity of 142°22'E. At the western end of the central basin (approximately 142°18'E), they recorded a depth of .
The western basin received four transects by "Stranger", recording depths of toward the central basin, near where "Trieste" dived in 1960 (vicinity , and where "Challenger II", in 1950, recorded . At the far western end of the "western" basin (about 142°11'E), the "Stranger" recorded , some 6 km south of the location where "Vityaz" recorded in 1957–1958. Fisher stated: "differences in the "Vitiaz" [sic] and "Stranger"–"Challenger II" depths can be attributed to the [sound] velocity correction function used". 
After investigating the Challenger Deep, "Stranger" proceeded to the Philippine Trench and transected the trench over twenty times in August 1959, finding a maximum depth of , and thus established that the Challenger Deep was about deeper than the Philippine Trench. The 1959 "Stranger" surveys of the Challenger Deep and of the Philippine Trench informed the U.S. Navy as to the appropriate site for "Trieste"'s record dive in 1960.
1962 – RV "Spencer F. Baird".
The "Proa Expedition, Leg 2", returned Fisher to the Challenger Deep on 12–13 April 1962 aboard the Scripps research vessel "Spencer F. Baird" (formerly the steel-hulled US Army large tug "LT-581") and employed a Precision Depth Recorder (PDR) to verify the extreme depths previously reported. They recorded a maximum depth of (location not available). Additionally, at location "H-4" in the Challenger Deep, the expedition cast three taut-wire soundings: on 12 April, the first cast was to 5,078 fathoms (corrected for wire angle) at in the central basin (Up until 1965, US research vessels recorded soundings in fathoms). The second cast, also on 12 April, was to 5,000+ fathoms at in the central basin. On 13 April, the final cast recorded 5,297 fathoms (corrected for wire angle) at (the western basin). They were chased off by a hurricane after only two days on-site. Once again, Fisher entirely missed the eastern basin of the Challenger Deep, which later proved to contain the deepest depths.
1975–1980 – RV "Thomas Washington".
The Scripps Institution of Oceanography deployed the 1,490-ton Navy-owned, civilian-crewed research vessel "Thomas Washington" (AGOR-10) to the Mariana Trench on several expeditions from 1975 to 1986. The first of these was the "Eurydice Expedition, Leg 8" which brought Fisher back to the Challenger Deep's western basin from 28–31 March 1975. "Thomas Washington" established geodetic positioning by (SATNAV) with Autolog Gyro and EM Log. Bathymetrics were by a 12 kHz Precision Depth Recorder (PDR) with a single 60° beam. They mapped one, "possibly two", axial basins with a depth of . Five dredges were hauled 27–31 March, all into or slightly north of the deepest depths of the western basin. Fisher noted that this survey of the Challenger Deep (western basin) had "provided nothing to support and much to refute recent claims of depths there greater than ." While Fisher missed the eastern basin of the Challenger Deep (for the third time), he did report a deep depression about 150 nautical miles east of the western basin. The 25 March dredge haul at encountered , which pre-shadowed by 22 years the discovery of HMRG Deep/Sirena Deep in 1997. The deepest waters of the HMRG Deep/Sirena Deep at are centered at/near , approximately 2.65 km from Fisher's 25 March 1975 dredge haul.
On Scripps Institution of Oceanography's "INDOPAC Expedition Leg 3", the chief scientist, Dr. Joseph L. Reid, and oceanographer Arnold W. Mantyla made a hydrocast of a free vehicle (a special-purpose benthic lander (or "baited camera") for measurements of water temperature and salinity) on 27 May 1976 into the western basin of the Challenger Deep, "Station 21", at at about depth. On "INDOPAC Expedition Leg 9", under chief scientist A. Aristides Yayanos, "Thomas Washington" spent nine days from 13–21 January 1977 conducting an extensive and detailed investigation of the Challenger Deep, mainly with biological objectives. "Echo soundings were carried out primarily with a 3.5 kHz single-beam system, with a 12 kHz echosounder operated in addition some of the time" (the 12 kHz system was activated for testing on 16 January). A benthic lander was put into the western basin () on 13 January,