The Challenger Deep is the deepest known point in the Earth's sea floor hydrosphere, with a depth of 10,898 m (35,755 ft) to 10,916 m (35,814 ft) by direct measurement from submersibles, and slightly more by sonar bathymetry (see below). It is in the Pacific Ocean, at the southern end of the Mariana Trench near the Mariana Islands group. 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. Its bottom is about 11 km (7 mi) long and 1.6 km (1 mi) wide, with gently sloping sides. The closest land to the Challenger Deep is Fais Island (one of the outer islands of Yap), 287 km (178 mi) southwest, and Guam, 304 km (189 mi) to the northeast. It is located in the ocean territory of the Federated States of Micronesia, 1 mi (1.6 km) from its border with ocean territory associated with Guam.
The depression is named after the British Royal Navy survey ship HMS Challenger, whose expedition of 1872–1876 made the first recordings of its depth. According to the August 2011 version of the GEBCO Gazetteer of Undersea Feature Names, the location and depth of the Challenger Deep are and 10,920 m (35,827 ft) ±10 m (33 ft).
June 2009 sonar mapping of the Challenger Deep by the Simrad EM120 (sonar multibeam bathymetry system for 300–11,000 m deep water mapping) aboard the RV Kilo Moana indicated a depth of 10,971 metres (35,994 ft; 6.817 mi). The sonar system uses phase and amplitude bottom detection, with a precision of better than 0.2% of water depth; this is an error of about 22 metres (72 ft) at this depth. Further soundings made by the US Center for Coastal & Ocean Mapping in October 2010 are in agreement with this figure, placing the deepest part of the Challenger Deep at 10,994 m (36,070 ft), with an estimated vertical uncertainty of ±40 m (130 ft).
Only four descents have ever been achieved. The first descent by any vehicle was by the manned bathyscaphe Trieste in 1960. This was followed by the unmanned ROVs Kaikō in 1995 and Nereus in 2009. In March 2012 a manned solo descent was made by the deep-submergence vehicle Deepsea Challenger. These expeditions measured very similar depths of 10,898 to 10,916 metres (35,755 to 35,814 ft).
History of depth mapping from the surface 
Over the years the search for the point of maximum depth has involved many vessels.
- The HMS Challenger expedition (December 1872 – May 1876) first sounded the depths now known as the Challenger Deep. This first sounding was made on 23 March 1875 at station 225. The reported depth was 4,475 fathoms (26,850 ft; 8,184 m) at , based on two separate soundings.
- A 1912 book, The Depths of the Ocean by Sir John Murray, records the depth of the Challenger Deep as 31,614 ft (9,636 m), reporting the sounding taken by the converted navy collier USS Nero in 1899. Murray was one of the expedition scientists.
- In 1951, about 75 years after its original discovery, the entire Mariana Trench was surveyed by a second Royal Navy vessel, captained by George Stephen Ritchie (later Rear Admiral Ritchie); this vessel was also named HMS Challenger, after the original expedition ship. This survey recorded the deepest part of the trench using echo sounding, a more precise and easier way to measure depth than the sounding equipment and drag lines used in the original expedition. A depth of 5,960 fathoms (35,760 ft; 10,900 m) was measured at .
- The maximum surveyed depth of the Challenger Deep was reported in 1957 by the Soviet vessel Vityaz recording a spot 11,034 metres (36,201 ft) ±50 m (164 ft) deep at  It was dubbed the Mariana Hollow and is listed in many reference sources, including the Encyclopædia Britannica, articles in National Geographic and on maps. The pressure at this depth is approximately 1,099 times that at the surface, or 111 MPa (16,099 psi). .
- In 1959 the US Navy research vessel RV Stranger using bomb-sounding surveyed a maximum depth of 10,915 m (35,810 ft) ±10 m (33 ft) at  .
- In 1962 the US Navy research vessel RV Spencer F. Baird using a frequency-controlled depth recorder surveyed a maximum depth of 10,915 m (35,810 ft) ±10 m (33 ft) at .
- In 1975 and 1980 the US Navy research vessel RV Thomas Washington using a precision depth recorder with satellite positioning surveyed a maximum depth of 10,915 m (35,810 ft) ±10 m (33 ft) at .
- In 1984, the survey vessel Takuyo from the Hydrographic Department of Japan, used a narrow, multi-beam echo sounder to take a measurement of 10,924 m (35,840 ft) ±10 m (33 ft) at .
- In 1998 a regional bathymetric survey of the Challenger Deep was conducted by the Deep Sea Research Vessel RV Kairei, from the Japan Agency for Marine-Earth Science and Technology, using a SeaBeam 2112 multibeam echosounder. The regional bathymetric map made from the data obtained in 1998 shows that the greatest depths in the eastern, central, and western depressions are 10,922 m (35,833 ft) ±74 m (243 ft), 10,898 m (35,755 ft) ±62 m (203 ft), and 10,908 m (35,787 ft) ±36 m (118 ft), respectively, making the eastern depression the deepest of the three.
- In 1999 and 2002 the RV Kairei revisited the Challenger Deep. The cross track survey in the 1999 RV Kairei cruise shows that the greatest depths in the eastern, central, and western depressions are 10,920 m (35,827 ft) ±10 m (33 ft), 10,894 m (35,741 ft) ±14 m (46 ft), and 10,907 m (35,784 ft) ±13 m (43 ft), respectively, which supports the results of the 1998 survey. The detailed grid survey in 2002 showed that the deepest site is located in the eastern part of the eastern depression around  , with a depth of 10,920 m (35,827 ft) ±5 m (16 ft), about 290 m (950 ft) southeast of the deepest site determined by the survey vessel Takuyo in 1984 and about 240 m (790 ft) east of the deepest place determined by the 1998 RV Kairei survey.
- On 1 June 2009 sonar mapping of the Challenger Deep by the Kongsberg Simrad EM 120 sonar multibeam bathymetry system for deep water (300 – 11,000 metres) mapping aboard the RV Kilo Moana (mothership of the Nereus underwater vehicle) indicated a depth of 10,971 m (35,994 ft). The sonar system uses phase and amplitude bottom detection, with an accuracy of better than 0.2% of water depth across the entire swath.
- On 7 October 2010 further sonar mapping of the Challenger Deep area was conducted by the US Center for Coastal & Ocean Mapping/Joint Hydrographic Center (CCOM/JHC) aboard the USNS Sumner (T-AGS-61). The results were reported in December 2011 at the annual American Geophysical Union fall meeting. Using a Kongsberg Maritime EM 122 multibeam echo sounder system coupled to positioning equipment that can determine latitude and longitude up to 50 cm (20 in) accuracy, from thousands of individual soundings around the deepest part the CCOM/JHC team determined that the Challenger Deep has a maximum depth of 10,994 m (36,070 ft) at , with an estimated vertical uncertainty of ±40 m (130 ft) at 2 standard deviations (≈ 95.4%) confidence level. A secondary deep with a depth of 10,951 m (35,928 ft) was located at approximately 23.75 nmi (44.0 km) to the east at in the Mariana Trench.
The 2009 and 2010 maximal depths were not confirmed by the series of dives Nereus made to the bottom during an expedition in May–June 2009. The direct descent measurements by the four expeditions which have reported from the bottom, have fixed depths in a narrow range from 10,916 m (Trieste) to 10,911 m (Kaikō), to 10,902 m (Nereus) to 10,898 m (Deepsea Challenger) Although an attempt was made to correlate locations, it could not be absolutely certain that Nereus (or the other descents) reached exactly the same points found to be maximally deep by the sonar/echo sounders of previous mapping expeditions, even though one of these echo soundings was made by Nereus mothership.
Manned descents 
On 23 January 1960, the Swiss-designed bathyscaphe Trieste, originally built in Italy and acquired by the U.S. Navy, descended to the ocean floor in the trench manned by Jacques Piccard (who co-designed the submersible along with his father, Auguste Piccard) and USN Lieutenant Don Walsh. Their crew compartment was inside a spherical pressure vessel, which was a heavy-duty replacement (of the Italian original) built by Krupp Steel Works of Essen, Germany. Their descent took almost five hours and the two men spent barely twenty minutes on the ocean floor before undertaking the three-hour-and-fifteen-minute ascent. Their early departure from the ocean floor was due to their concern over a crack in the outer window caused by the temperature differences during their descent. The measured depth at the bottom was measured with a manometer at 10,916 m (35,814 ft) ±5 m (16 ft).
Deepsea Challenger 
On 26 March 2012 (local time), Canadian film director James Cameron made a solo manned descent in the DSV Deepsea Challenger to the bottom of the Challenger Deep. At approximately 05:15 ChST on 26 March (19:15 UTC on 25 March), the descent began. At 07:52 ChST (21:52 UTC), the Deepsea Challenger arrived at the bottom. The descent lasted 2 hours and 36 minutes and the recorded depth was 10,898.4 metres (35,756 ft) when the Deepsea Challenger touched down. Cameron planned to spend about six hours near the ocean floor exploring but, due to a hydraulic fluid leak in the lines controlling the manipulator arm which obscured the visibility out the only viewing port in the DSV and caused the loss of the submersible's starboard thrusters, decided to start the ascent to the surface after 2 hours and 34 minutes exploring the ocean floor. At around 12:00 ChST (02:00 UTC on 26 March), the Deepsea Challenge website says the sub resurfaced after a 90-minute ascent, although Paul Allen's tweets indicate the ascent took only about 67 minutes. During a post-dive press conference Cameron said: "I landed on a very soft, almost gelatinous flat plain. Once I got my bearings, I drove across it for quite a distance ... and finally worked my way up the slope." The whole time, Cameron said, he didn't see any fish, or any living creatures more than an inch (2.5 cm) long: "The only free swimmers I saw were small amphipods"—shrimplike bottom-feeders.
Planned manned descents 
Several other manned expeditions are planned. These include:
- Triton Submarines, a Florida based company that designs and manufactures private submarines, whose vehicle, Triton 36000/3, will carry a crew of three to the seabed in 120 minutes;
- Virgin Oceanic, sponsored by Richard Branson's Virgin Group, is developing a submersible designed by Graham Hawkes, DeepFlight Challenger, with which the solo pilot will take 140 minutes to reach the seabed;
- DOER Marine, a San Francisco Bay Area based marine technology company established in 1992, that is developing a vehicle, Deepsearch (and Ocean Explorer HOV Unlimited), with some support from Google's Eric Schmidt with which a crew of two or three will take 90 minutes to reach the seabed, as the program Deep Search.
Unmanned descents 
On 24 March 1995, the Japanese robotic deep-sea probe Kaikō broke the depth record for unmanned probes when it reached close to the surveyed bottom of the Challenger Deep. Created by the Japan Marine Science and Technology Center (JAMSTEC), it was one of the few unmanned deep-sea probes in operation that could dive deeper than 6,000 metres (20,000 ft). The manometer measured depth of 10,911 m (35,797 ft) ±3 m (10 ft) at  for the Challenger Deep is believed to be the most accurate measurement taken yet. Kaikō also collected sediment cores containing marine organisms from the bottom of the deep. Kaikō made many unmanned descents to the Mariana Trench during three expeditions in 1995, 1996 and 1998. The greatest depth measured by Kaikō in 1996 was 10,898 m (35,755 ft) at and in 1998 10,907 m (35,784 ft) at .
On 31 May 2009 the United States sent the Nereus hybrid remotely operated vehicle (HROV) to the Challenger Deep. Nereus thus became the first vehicle to reach the Mariana Trench since 1998 and the deepest-diving vehicle currently in operation. Project manager and developer Andy Bowen heralded the achievement as "the start of a new era in ocean exploration". Nereus, unlike Kaikō, did not need to be powered or controlled by a cable connected to a ship on the ocean surface.
Nereus spent over 10 hours at the bottom of the Challenger Deep and measured a depth of 10,902 m (35,768 ft) at RV Kilo Moana at the surface and collecting geological and biological samples from the Challenger Deep bottom with its manipulator arm for further scientific analysis., while sending live video and data back to its mothership
The Nereus is operated by the Woods Hole Oceanographic Institution.
The Summary Report of the HMS Challenger expedition lists radiolaria from the two dredged samples taken when the Challenger Deep was first discovered. These (Nassellaria and Spumellaria) were reported in the Report on Radiolaria (1887) written by Ernst Haeckel.
"... And as we were settling this final fathom, I saw a wonderful thing. Lying on the bottom just beneath us was some type of flatfish, resembling a sole, about 1 foot long and 6 inches across. Even as I saw him, his two round eyes on top of his head spied us — a monster of steel — invading his silent realm. Eyes? Why should he have eyes? Merely to see phosphorescence? The floodlight that bathed him was the first real light ever to enter this hadal realm. Here, in an instant, was the answer that biologists had asked for the decades. Could life exist in the greatest depths of the ocean? It could! And not only that, here apparently, was a true, bony teleost fish, not a primitive ray or elasmobranch. Yes, a highly evolved vertebrate, in time's arrow very close to man himself. Slowly, extremely slowly, this flatfish swam away. Moving along the bottom, partly in the ooze and partly in the water, he disappeared into his night. Slowly too — perhaps everything is slow at the bottom of the sea — Walsh and I shook hands.
Many marine biologists are now skeptical of this supposed sighting, and it is suggested that the creature may instead have been a sea cucumber. The video camera on board the Kaiko probe spotted a sea cucumber, a scale worm and a shrimp at the bottom. At the bottom of the Challenger deep, the Nereus probe spotted one polychaete worm (a multi-legged predator) about an inch long.
An analysis of the sediment samples collected by Kaiko found large numbers of simple organisms at 10,900 m (35,800 ft). 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 represent taxa distinct from those in shallower ecosystems.
Most of the organisms collected were simple, soft-shelled foraminifera (432 species according to National Geographic), with four of the others representing species of the complex, multi-chambered genera Leptohalysis and Reophax. Eighty-five percent of the specimens were organic, soft-shelled allogromiids, which 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%. As small organisms with hard, calcareous shells have trouble growing at extreme depths because of the high solubility of calcium carbonate in the pressurized water, scientists theorize that the preponderance of soft-shelled organisms in 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 species that survived the change in depth were the ancestors of the Challenger Deep's current denizens.
On 17 March 2013, researchers reported data that suggested microbial life forms thrive in the Mariana Trench, the deepest spot on the Earth. Other researchers reported related studies that microbes thrive inside rocks up to 1900 feet below the sea floor under 8500 feet of ocean off the coast of the northwestern United States. According to one of the researchers,"You can find microbes everywhere — they're extremely adaptable to conditions, and survive wherever they are."
See also 
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- The Colbert Report, airdate: 2012 April 12, interview with James Cameron
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- Page 131 of Murray's book refers to the Challenger Deep.
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- "Life Is Found Thriving at Ocean's Deepest Point", National Geographic News, 3 February 2005
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- EM 120 sonar multibeam bathymetry system brochure
- EM 122 Multibeam echosounder
- EM 122 sonar multibeam bathymetry system brochure
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- National Geographic (25 March 2012). "James Cameron Now at Ocean's Deepest Point". National Geographic Society. Retrieved 25 March 2012.
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- Revisiting the Challenger Deep using the ROV Kaiko
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- "'Nereus' reaches deepest part of the ocean". physorg.com. 2 June 2009. Retrieved 2 June 2009.
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