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Looking into Devils Hole; the dark area is the surface of the water.

Devils Hole is a geologic formation located in a detached unit of Death Valley National Park and surrounded by the Ash Meadows National Wildlife Refuge, in Nye County, Nevada, in the Southwestern United States.

Devils Hole is habitat for the only naturally occurring population of the endangered Devils Hole pupfish (Cyprinodon diabolis). The 40 acre unit is part of the Ash Meadows complex, an area of desert uplands and spring-fed oases that was designated as a National Wildlife Refuge in 1984.

Description
Devils Hole is a geothermal pool within a limestone cavern in the Amargosa Desert in the Amargosa Valley of Nevada, east over the Amargosa Range and Funeral Mountains from Death Valley. It is at an elevation of 730 m above sea level and the water is a constant temperature of 33 C. The surface area of Devils Hole is about 22 m long by 3.5 m wide (72 ft long by 11.5 ft wide). Approximately 0.3 m deep on one end of Devils Hole is a small rock shelf of 3.5 by. The dissolved oxygen of the water is 2.5–3.0 ppm up to around 22 m in depth, though the shallow shelf can have dissolved oxygen levels as high as 6.0–7.0 ppm in June and July.

Devils Hole branches into caverns at least 130 m deep, whose bottom has never been mapped. According to geologists, the caves were formed over 500,000 years ago. The pool has frequently experienced activity due to far away earthquakes in Japan, Indonesia, Mexico, and Chile, which have been likened to extremely small scale tsunamis.

Below the surface pool, Devils Hole descends approximately 160 ft through what is termed the "main chamber" before reaching a narrow opening referred to as the 'funnel'. Through this opening lies a much larger chamber of the cavern system known as Acree's Chasm. Acree's Chasm is approximately 300 ft in length, 40 ft in width, and has a bottom approximately 260 ft below the surface.

Immediately after passing the funnel into Acree's Chamber, a narrow side tube can be found to a diver's left. This side tube is narrow and proceeds approximately 90 ft upward to a chamber with an air pocket, named Brown's Room after its discoverer William Brown in 1953. The tube leading to Brown's Room has at least 2 offshoots, the higher of which leads to a dead-end filled with a small air pocket, and the lower of which confluences with additional tubes descending from Brown's Room. If the diver instead descends through Acree's Chamber, the first notable landmark is a rocky shelf termed the 'lower ledge,' around 100 ft below the entrance to the chamber. The bottom of Acree's Chamber lies around 260 ft below the surface, but is not flat. Instead, a portion of the chamber floor descends below this lower shelf; a gradual funnel leads to a hole in the bottom of the chamber featuring a strong current. The hole, later termed the ojo de agua, is 315 ft below the surface and just large enough for a diver with equipment to fit through. However, on June 20th, 1965, during the second dive of a rescue and then body recovery mission, Jim Houtz with his dive partner, dropped a weighted depth line to a depth of 932 ft from the start of this opening, without hitting the bottom of the chamber below. Due to the strong current, small size of the entrance, and unknown depth of the below cavern Houtz termed the "Infinity Room," Jim and his partner chose not to explore this Infinity Room. This mission did, however, confirm that the depth of the Infinity Room of Devil's Hole, and the cavern system itself, has a depth of at least 1247 ft from the surface. A subsequent USGS exploration into Devils Hole in 1991 by Alan Riggs, Paul DeLoach, and Sheck Exley entered what they found out to be a narrow tube rather than an 'Infinity Room' at 315 ft, descending to a depth of 436 ft. The team reported being able to see down to a depth of some 500 ft, without visualizing the bottom of the cavern.

Geology
Dense, white, billowy coatings of mammillary calcite originally precipitated on all of Devils Hole’s underwater surfaces, as calcium-carbonate-supersaturated aquifer water circulated through it. The mammillary calcite coating is as much as 100 cm thick in places and is the material that has been analyzed to develop a continuous 800,000-year-long continental paleoclimate record.

Other calcium carbonate morphologies, including folia and flowstone, precipitated in Devils Hole, but in different locations and under different circumstances. Folia precipitate on the walls of Brown’s Room, a perpetually dark air-filled chamber. Foliar growth is stimulated by Devils Hole’s mixed semidiurnal tide (caused by small changes in aquifer volume as the moon’s and sun’s gravitational forces cyclically distort the aquifer) that alternately wets and exposes a small band of wall around mean water level twice a day. Maximum spring tide amplitude is about 12 cm.

A 7.4-magnitude 2012 Guerrero–Oaxaca earthquake, some 2000 mi away, centered roughly 12 mi below the surface, on March 20th, 2012, caused an undulating 4 ft rise and fall of the cavern waters, as appreciated by researchers working at Devils Hole at the time. This provided further evidence that Devils Hole cave system was connected to not only the Death Valley Regional Groundwater Flow System, but possibly to even further-reaching underground water systems. The 1991 USGS dive team described the Devils Hole as a "skylight" into the water table.

Devils Hole Cave
Located 650 ft north of Devils Hole is a separate cave system called Devils Hole Cave (#2). It was first explored underwater to a depth of 70 ft by divers from he Southwestern Speleological Society in February of 1961. It had been described as being shaped like a boot with fallen rock restriction at the 50 ft level leading to a narrow pool of 93 F water. Since no sunlight reaches the water, algae cannot grow and no fish species are found.

In 1991, deep diver Sheck Exley located a bottom to the eastern shaft at 128.9 m

On the surface, the cave openings are connected to Devils Hole by an access road and covered with a locked metal grate Below ground, a passable deepwater connection to Devils Hole has been theorized but remains undiscovered.

A team of paleoclimatologists from the University of Innsbruck have been collecting and dating calcite mineral deposits here since 2010. In March of 2017, underwater cinematographer Jonathan Bird received permission to assist scientists in a four day expedition to take water and calcite core samples. IMAX footage was included in the 2020 film Ancient Caves and extra footage was used to create the video documentary. Similar studies had been done in Devils Hole but are no longer allowed permitted due to the endangered status of the Devils Hole pupfish.

Pupfish
Devils Hole is the only natural habitat of the Devils Hole pupfish, which thrives despite the hot, oxygen-poor water. Devils Hole "may be the smallest habitat in the world containing the entire population of a vertebrate species". The pupfish are considered critically endangered by the IUCN. The pupfish has been described as the world's rarest fish, with a population of less than 200 since 2005. Genetic information indicates that the pupfish species is as old as the Hole itself, which opened to the surface about 60,000 years ago.

The pupfish have been protected since being declared an endangered species in 1967. Conflicts of the ownership and use of the groundwater around Devils Hole caused litigation in the 1980s. The litigation triggered further protections of the pupfish. However, since the late 1990s, the pupfish population has substantially decreased. The reasons for the decrease are unknown, but is possibly due to a microscopic non-indigenous diving beetle that is consuming pupfish eggs.