The Underground: Caves and What Lies Beneath

The Entrance Chamber

Rainwater picks up carbon dioxide, turns mildly acidic, dissolves limestone, then drops the calcite again when it hits cave air. One grows down from the ceiling, one grows up from the floor, and in many caves the whole process manages about a cubic centimetre a century.

• Deep Earth

The Entrance Chamber

Kentucky's limestone labyrinth is the longest surveyed cave on Earth, past 686 km and still lengthening. The 1972 connection between the Flint Ridge system and Mammoth put it so far ahead of every rival that nothing has come close since.

• Record Depth
• Deep Earth

The Entrance Chamber

A New Mexico cave dissolved not by rainwater but by sulfuric acid rising from oil and gas deposits below. The Big Room is one of the largest single cave chambers in North America.

• Deep Earth

The Entrance Chamber

Hundreds of thousands of Brazilian free-tailed bats spiral out of Carlsbad's natural entrance at dusk each summer. The park bans electronics during the flight so the colony leaves on its own schedule rather than ours.

• Deep Earth

The Entrance Chamber

Found by four teenagers in the Dordogne in 1940, then shut to the public in 1963 when visitors' breath began growing algae and crystals on the walls. The bulls tourists queue for today are a millimetre accurate replica built next door.

• Cave Art
• Sealed Off

Past the Handrails

The largest known cave passage on the planet, in Vietnam's Phong Nha-Ke Bang. Collapsed skylights let in enough sun to grow a jungle inside it, and the chamber is big enough to generate its own clouds.

• Record Depth
• Deep Earth

Past the Handrails

Miners at Naica, Mexico broke into a chamber of gypsum beams up to 11 metres long and 55 tonnes each, grown in water held near 58 C for millennia. It is only accessible because pumps keep it drained; stop pumping and it floods and resumes growing.

• Deep Earth
• Sealed Off

Past the Handrails

Boats drift under a ceiling of Arachnocampa luminosa larvae in New Zealand's Waikato. The lights are bait: each larva dangles sticky fishing lines to catch insects drawn to the glow.

• Deep Earth
• Extremophiles

Past the Handrails

The olm, a blind cave salamander in Slovenia, can live a century, regrow limbs and go years without food. Floods used to wash them out of the karst, which is how locals concluded the caves were breeding dragons.

• Extremophiles
• Myth

Past the Handrails

Soviet drilling on the Kola Peninsula reached 12,262 m before heat near 180 C made the rock behave like plastic and stopped the project in 1992. It is still the deepest artificial point on Earth, and the hole is now welded shut.

• Record Depth
• Deep Earth
• Sealed Off

The Dark Zone

The deepest known cave on Earth, in Abkhazia's Arabika Massif. In 2018 a flood pulse hit an expedition camped 2,212 m down and the team climbed out through a waterfall pouring over their own ropes, abandoning most of their gear.

• Record Depth
• Deep Earth

The Dark Zone

Also called Voronya, on the same Abkhazian massif. It was the first cave confirmed past 2,000 m and held the world depth record for years before its neighbour Veryovkina took it.

• Record Depth
• Deep Earth

The Dark Zone

Cavers dug through rubble in 1986 and found 233 km of gypsum chandeliers and sulfur formations documented nowhere else. It is closed to all but approved research trips, partly because its rock-eating microbes are of medical interest.

• Sealed Off
• Extremophiles
• Deep Earth

The Dark Zone

The largest cave chamber in the world by floor area, roughly 600 m by 435 m, inside Borneo's Gunung Mulu. Reaching it means a long trek and a passage up an underground river, and no lamp will show you the far wall.

• Record Depth
• Deep Earth

The Dark Zone

Spain's 22,000-year-old painted bison were shut away in 1977 after tourism began degrading them. In 2014 the site began letting in five randomly chosen visitors a week for under 40 minutes, with sensors watching the air the whole time.

• Cave Art
• Sealed Off

No Sunlight

A Romanian cave sealed under impermeable clay for roughly 5.5 million years, found in 1986. More than fifty invertebrate species live there on chemosynthesis fed by hydrogen sulfide and methane, and most exist nowhere else on Earth.

• Extremophiles
• Sealed Off
• Deep Earth

No Sunlight

In Cueva de Villa Luz, Tabasco, colonies of sulfur-eating bacteria hang from the ceiling like mucus and drip sulfuric acid at roughly the pH of battery acid. The air is lethal to enter unprotected.

• Extremophiles
• Deep Earth

No Sunlight

A South African chamber reachable only through an 18 cm squeeze, which is why the excavation team was recruited for build rather than seniority. The Homo naledi remains inside may be deliberate burials from a small-brained species, which would rewrite who buries their dead.

• Human Remains
• Deep Earth

No Sunlight

A single fragment of finger bone from a Siberian cave produced a genome belonging to no known human species. The Denisovans were identified from DNA before anyone had a skull to look at.

• Human Remains
• Deep Earth

No Sunlight

A drowned pit deep inside Yucatan's flooded cave systems, holding the near-complete skeleton of a teenage girl who fell in some 13,000 years ago. Her skull looks nothing like modern Native Americans, but her DNA says she is their ancestor.

• Human Remains
• Deep Earth

Sumps And Squeezes

The deepest cave in the Western Hemisphere, 1,560 m down and 85 km long under Oaxaca. Expeditions ask permission of the Mazatec, for whom these entrances are not sport routes but gateways.

• Record Depth
• Deep Earth

Sumps And Squeezes

A 1990 dye trace showed water entering Cheve resurfacing far below and far away, meaning the system could be the deepest on Earth if anyone can push the sumps. Decades of expeditions have not yet connected it.

• Record Depth
• Deep Earth

Sumps And Squeezes

Nearly 400 snapped-off stalagmites arranged into circles 336 m inside a French cave, dated to about 176,000 years ago. Neanderthals built them, deep in the dark with fire, and nobody knows what for.

• Human Remains
• Deep Earth

Sumps And Squeezes

Under the Nullarbor, a flooded passage that made Australia a cave-diving destination. Expeditions from the 1970s onward pushed kilometres past the last air, hauling gear on underwater sleds through water with no exit but the way in.

• Record Depth
• Deep Earth

Sumps And Squeezes

Elephants walk into this Kenyan cave at night and mine the walls with their tusks for salt. Two visitors, in 1980 and 1987, came out with Marburg virus and died; the cave became the opening of "The Hot Zone".

• Extremophiles
• Deep Earth

Bottom of the Hole

Pinned by a rock in Sand Cave, Kentucky, in January 1925 while hunting a new entrance to Mammoth. The seventeen-day failed rescue drew thousands of spectators, a carnival midway and one of America's first media circuses; he died before they reached him.

• Human Remains
• Sealed Off

Bottom of the Hole

John Edward Jones wedged head-down in an unmapped Utah crevice in November 2009 and died after some 28 hours. Recovery was judged too dangerous, so the entrance was filled with concrete with him still inside; the cave is now his grave.

• Human Remains
• Sealed Off

Bottom of the Hole

Juan Moricz claimed an Ecuadorian cave held a "metallic library" of gold tablets from a lost civilisation, a claim Erich von Daniken made famous. The real 1976 expedition, with Neil Armstrong as honorary president, found no gold, no tablets and no library.

• Myth
• Hoax
• Deep Earth

Bottom of the Hole

In 1997 a caller named Mel Waters told Art Bell's radio audience about a bottomless pit near Ellensburg, Washington, and a dead dog thrown in that came back alive. No hole, no property record and no Mel Waters has ever been found; treat it as a hoax.

• Hoax
• Myth

Bottom of the Hole

A 1989 story claimed Soviet drillers broke into a cavity and recorded the screams of the damned. It is a fabrication that travelled from Finland through American religious broadcasters and back; the real borehole hit 180 C and no cavity at all.

• Hoax
• Deep Earth

Bottom of the Hole

The hollow earth began as a serious 17th-century hypothesis from Edmond Halley, then curdled through Symmes, occultists and Thule Society mysticism into a hidden inner world with polar entrances. Seismology settled it long ago; the belief did not notice.

• Myth
• Deep Earth