Challenger Deep

• 1. Introduction

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  The Challenger Deep lies in the western Pacific Ocean, at the southern end of the Mariana Trench, inside the ocean territory of the Federated States of Micronesia. It is the deepest known point on Earth's seabed. A 2021 study put its depth at 10,935 metres, give or take 6, at a 95 percent confidence level. That tidy figure hides a long argument. For nearly a century and a half, ships have lowered rope, wire, explosives and sound pulses into this slot, and each generation has come back with a slightly different answer. As of 2022, only 27 people have travelled down to it. Why did a place this deep stay hidden for so long? Why did survey after survey miss its deepest pool? And why can scientists still not agree on exactly where the bottom lies?

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• 2. Three Pools

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  Three separate basins, each between 6 and 10 kilometres long and about 2 kilometres wide, make up the Challenger Deep. They sit side by side in a staggered line running west to east, like shingles on a roof. Mounds between them rise 200 to 300 metres higher than the basin floors. Measured along the 10,650-metre contour, the whole formation stretches about 48 kilometres.

  This slot-shaped depression is itself tucked inside a much larger crescent-shaped trench, which is already an unusually deep feature of the ocean floor. Sonar readings show the western and eastern basins both passing 10,920 metres, while the centre basin is slightly shallower.

  It is remote even by the standards of the open ocean. The nearest land is Fais Island, one of the outer islands of Yap, 155 nautical miles to the southwest. Guam lies 164 nautical miles to the northeast.

  The idea that these are three distinct deeps came from Japanese surveys of 1998 and 1999 aboard the research vessel Kairei. Their authors proposed calling them the East, Central and West Deep. Later work in 2002 trimmed the basins to their present dimensions.

  The floor itself is not the flat, silent plain that early charts implied. In June 2020, the ship DSSV Pressure Drop mapped all three basins with sonar, and crewed descents added a close look. They found slopes and piles of rock rising from a bed of fine pelagic ooze.

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• 3. Two Ships Named Challenger

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  Baffling winds sent the British corvette HMS Challenger off course in 1875. She was sailing from the Admiralty Islands toward Yokohama and had hoped to reach Guam. Instead the wind pushed her west, and her crew could not visit either the Carolines or the Ladrones. That detour carried the ship over the undersea canyon that would one day bear her name.

  The crew measured depth the slow way, with a marked rope fitted with a Baillie weight. They fixed their position by the stars, accurate to roughly 2 nautical miles. On the 23rd of March 1875, at sampling station 225, they logged a bottom of 4475 fathoms and then confirmed it with a second sounding at the same spot. It was the deepest reading of their three-year voyage around the world. One sample fell within fifteen miles of the deepest point in all of Earth's oceans.

  This was luck, and the source treats it as such. The expedition was history's first major scientific voyage devoted entirely to the young science of oceanography. By comparison, the Sirena Deep, the third deepest known site and only 150 nautical miles to the east, stayed undiscovered for another 122 years.

  Seventy-five years later, a 1,140-ton British survey ship carrying the same name came back. On her way south from Japan to New Zealand in 1951, HMS Challenger II ran bomb soundings across the trench. Her echo sounder could not confirm the result, so the crew paid out a taut wire weighted with 140 pounds of scrap iron.

  Thomas Gaskell, the senior scientist aboard, remembered the wait. He wrote that it took from ten past five in the evening until twenty to seven, an hour and a half, for the iron weight to fall to the sea bottom. It was almost dark when it struck, he said, but great excitement greeted the reading.

  In New Zealand, the Royal New Zealand Dockyard boosted the echo sounder so it could record at greater depths. The ship returned in October 1951 and found a broad area deeper than 5900 fathoms, which later proved to be the western basin. The name Challenger Deep honours both vessels. Gaskell thought it fitting that a ship called Challenger should put the seal on the pioneers' work.

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• 4. The Basin Everyone Missed

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  In August 1957, the Soviet research vessel Vityaz crossed the western basin and logged a depth of about 11,000 metres. She returned in 1958 for a detailed survey. Those readings were later treated as statistical outliers, and a depth greater than 11,000 metres has never been proven.

  The first firm confirmation came in July 1959 from the 325-ton research vessel Stranger. Dr. R. L. Fisher of the Scripps Institution of Oceanography led the work, using explosive soundings. The ship also crossed the Philippine Trench more than twenty times and found it about 800 metres shallower, which helped the U.S. Navy choose a site for its record dive.

  But Stranger's zigzag track passed well to the east of the eastern basin on one leg and well to the west on the other. She never saw it. That gap mattered, because the eastern basin would later be reported as deeper than the other two.

  Fisher missed it again in April 1962 aboard the Spencer F. Baird, when a hurricane chased the ship away after only two days. He missed it a third time in March 1975 aboard the Thomas Washington. That cruise did turn up an odd 10,015-metre dredge haul about 150 nautical miles east, a hint of the Sirena Deep 22 years before it was found.

  The Thomas Washington kept coming back through 1977-1978 and 1980. Its chief scientist, A. Aristides Yayanos, was chasing biology, so the ship concentrated on the western and central basins. On one pass in 1978 it recorded depths of only 5,093 to 7,182 metres south and west of the eastern basin. The source calls it simply another miss.

  The answer came from a new kind of sonar. The Japanese survey ship Takuyo, newly commissioned in 1984, carried the first narrow-beam SeaBeam multibeam system in Japanese service. Her crew had to write their own software to turn the digital data into charts. In three days they covered about 140 square kilometres and recorded 10,920 metres, giving the first documentation of the eastern basin as deepest. In 1993, GEBCO recognised that figure as the deepest depth of the world's oceans.

  In October 2002, the Kairei checked it with ten parallel tracks, north-south and east-west, less than 250 metres apart. The deepest point sat about 290 metres southeast of Takuyo's spot.

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• 5. Life at the Bottom

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  In January 1977, a camera-equipped lander sat on the floor of the western basin for 50 hours. When it came up, damaged but repairable, it carried excellent photographs of amphipods, small shrimplike crustaceans. The lander was put down once more, in the central basin, and never came back. It may still be there.

  On the 21st of November 1980, Yayanos achieved the first successful recovery of a live animal from the Challenger Deep. A pressurised trap brought up an amphipod from about 10,900 metres. Keeping the animal under pressure on the way up was the key to getting it alive.

  The 2002 Kairei expedition sent the robot Kaikō down in repeated dives to service landers. Traps came up full of amphipods, which the source also calls sea fleas. Cameras recorded sea cucumbers, white bristle worms and tube worms.

  One species keeps appearing in the records. In November 2013, a Japanese team lowered a lander carrying eleven baited traps into the central basin. After less than nine hours at the bottom, they hauled up about 90 individuals of the large amphipod Hirondellea gigas, collected to study their DNA and RNA.

  A Chinese lander mission in December 2016 reached depths past 10,000 metres and returned with 103 amphipods in its third device. Researchers from the Hadal Life Science Research Center at Shanghai Ocean University led the work.

  Fish live on the slopes above. On the 29th of January 2017, baited traps on the northern slope at 7,581 metres produced photographs and samples of a snailfish new to science. It was named Pseudoliparis swirei.

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• 6. Instruments Under Pressure

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  On the 14th of May 2017, the Japanese robot UROV11K began its ascent from 10,899 metres in the eastern basin. At 5,320 metres its buoyancy failed. Every effort to recover the vehicle was unsuccessful, and a machine built for full ocean depth stayed in the trench.

  In December 2014, the Schmidt Ocean Institute's research vessel Falkor released four untethered instruments into the central basin. The probe Deep Sound 3 imploded at 8,620 metres, about 2,200 metres above the bottom. A second probe, Deep Sound 2, was floating nearby and captured the sound. That gave scientists a rare recording of an implosion inside the Challenger Deep. A water-chemistry lander named ARI never answered the signal to drop its weights and was never recovered.

  Some deployments went the other way. In July 2015, the Coast Guard cutter Sequoia lowered a 45-metre mooring of hydrophones and pressure sensors into the western basin, a six-hour descent. It sat at 10,854.7 metres for 16 weeks. The sensors heard earthquakes, whale and dolphin calls, ship propellers, airguns, active sonar and a passing Category 4 typhoon.

  The longest wait belonged to a 7-kilometre mooring left by the Dutch institute NIOZ during the Sonne expedition of November 2016. Its acoustic release failed when a ship tried to retrieve it in 2018. In November 2019, the research vessel Sally Ride lowered a special release unit to about 1,000 metres, and it managed to reach the near-bottom releases.

  The mooring held 395 temperature sensors, the lowest only 8 metres above the trench floor. After almost three years underwater, 15 of them had mechanical problems. Early results from the rest pointed to internal waves moving through the Challenger Deep.

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• 7. Going Down in Person

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  A crack in an outer window cut short the first crewed visit. On the 23rd of January 1960, the bathyscaphe Trieste reached the western basin with Jacques Piccard and U.S. Navy Lieutenant Don Walsh aboard. Piccard had co-designed the vessel with his father, Auguste. Its crew sphere, 2.16 metres across, had steel walls 12.7 centimetres thick, built by Krupp in Essen, Germany.

  The descent took almost five hours. The two men spent barely twenty minutes on the bottom, worried by the crack that temperature differences had caused. The ascent took another three hours and fifteen minutes. An onboard manometer read 10,911 metres, give or take 7.

  It was 52 years before anyone else went down. On the 26th of March 2012, Canadian film director James Cameron dived alone in the Deepsea Challenger. The trip down took 2 hours and 36 minutes, and the sub touched bottom at 10,908 metres.

  Cameron had planned about six hours of exploring and stayed only 2 hours and 34 minutes. A hydraulic fluid leak blocked the view from his only window and cost the sub its starboard thrusters. At a press conference he described a very soft, almost gelatinous flat plain. He said the only free swimmers he saw were small amphipods.

  The Five Deeps Expedition set out to map and visit the deepest points of all five oceans. On the 28th of April 2019, Victor Vescovo descended into the eastern pool in the submersible Limiting Factor. Four dives followed between the 28th of April and the 4th of May, the last one to the slightly shallower central pool. The pressure reading at 10,928 metres of seawater depth was 1126.79 bar.

  A technical fault left the unmanned lander Skaff stuck on the bottom for two and a half days before Limiting Factor salvaged it. In 2020, the Ring of Fire expedition added six crewed descents and 25 lander deployments across all three basins.

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• 8. Why the Number Moves

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  A sonar pulse sent to the bottom of the Challenger Deep spreads out as it falls. A 2.7-degree SeaBeam ping from the Hakuhō Maru in 1992 covered a circle about 500 metres wide at 11,000 metres depth. A dip narrower than that is hard to see from a ship seven miles above.

  Sound itself is another problem. How fast it travels depends on water temperature, pressure and dissolved salt, and these vary with depth. Even the Yokosuka's accuracy of about 0.1 percent of water depth amounted to roughly 110 metres at the bottom of the trench.

  The disagreement sharpened with a 2014 study by James V. Gardner and colleagues at the University of New Hampshire. They reanalysed 2,051,371 soundings from eight survey lines run in 2010 aboard USNS Sumner. Their deepest point was 10,984 metres, with an uncertainty of 25 metres.

  The Sonne expedition in 2016 and the Sally Ride expedition in 2019 both objected. Hans van Haren, chief scientist on the Sally Ride, said Gardner's figure was 69 metres too deep because of the sound velocity profile used. The Sonne team had calibrated theirs with CTD casts about 1.6 kilometres west of its deepest sounding.

  A 2021 data paper led by Cassandra Bongiovanni reported 10,924 metres in the eastern basin, close to the Sonne value. Yet its position was about 42 kilometres from the Sonne location, and the paper found a nearly equal point in the western basin. It concluded that low-frequency ship sonar and deep-sea pressure sensors lack the accuracy to say which spot is truly deepest.

  The pressure-based study by Samuel Greenaway, Kathryn Sullivan and colleagues drew on submersible dives in June 2020. It corrected for gravity, water properties and atmospheric pressure. Their error term is dominated by the pressure sensor itself, and they found the western basin very nearly as deep as the eastern. Direct pressure readings stay in a narrow band. Limiting Factor logged 10,923 metres in the western basin and 10,915 in the central basin, each give or take 4.

  Published depths since 2000 still differ by more than their stated margins of error. That gap leaves open questions about the measurements or about how uncertainty has been reported. A separate 2021 paper by Loranger, Barclay and Buckingham, which estimated a depth of 10,983 metres from an implosion shock wave, examined the same disagreement.

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