Deer Cave with show cave path, largest cave passage in the world, Gunung Mulu National Park, Sarawak, island of Borneo, Malaysia

Upper Antelope, a slot canyon, Arizona, United States of America (U.S.A.), North America

Cleveland Avenue passage in Mammoth Cave System, Kentucky, United States of America, North America

Interior of the Gruta do Janelao, a limestone cave passage 100m high, lit through roof window, at Peruacu in Minas Gerais state, Brazil, South America

A man exploring the old abandoned Coniston copper mines in the Lake District, Cumbria, England, United Kingdom, Europe

A caver in a tight squeeze in the Yorkshire Dales, England, United Kingdom, Europe

Cavers in County Pot in the Yorkshire Dales, Yorkshire, England, United Kingdom, Europe

A caver in the Yorkshire Dales, Yorkshire, England, United Kingdom, Europe

Tunnel through a glacier, Gletscherpalast at Kleines Matterhorn (3883 m), Zermatt, Wallis, Valais, Switzerland

Entrance Quackenschloss, passage cave, karst cave, geotope, natural monument, near Engelhardsberg, district of Markt Wiesenttal, Upper Franconia, Franconian Switzerland, Franconia, Bavaria, Germany, Europe

Quackenschloss, passage cave, karst cave, geotope, natural monument, near Engelhardsberg, district of Markt Wiesenttal, Upper Franconia, Franconian Switzerland, Franconia, Bavaria, Germany, Europe

Rock gate Geisskirche, passage cave, mystical, hamlet Tuerkelstein, district of Goessweinstein, Upper Franconia, Franconia, Bavaria, Germany, Europe

Historic architecture with bells and arcades, embedded in rocky structures, aerial view, Church of the Assumption, cave town, cave monastery, Vardzia, Vardzia, Samtskhe-Javakheti region, Samtske Javakheti, Lesser Caucasus, Georgia

Undergroun caves, Camerano, Marche, Italy

A cave explorer poses in a tube in a cave in Mulu National Park

Katie Dent climbs up a rope in a cave in the White Mountains on Crete.

Cave explorers add scale to a cave in Mulu National Park

A cave explorer has to remove his helmet in order to pass the tight squeeze in an entrance to a cave in the White Mountains on t

A cave explorer on a speleothem at the first doline, or skylight, in Hang Son Doong while two other explorers illuminate a large passage leading to the second doline 1.5 kilometers away.

A cave explorer illuminates a large passage looking out towards the second doline in Hang Son Doong, Vietnam.

Cave explorers illuminate a large passage in Hang En, Phong Nha Ke Bang National Park, Vietnam.

Cave explorer ascends a knotted rope out above a pool of water in a cave in England called Streaks Pot.

Cave explorer struggles to negotiate a tight squeeze in the entrance of a cave in the White Mountains on the island of Crete.

A cave explorer admires pretty cave formations in Mulu National Park.

Cave explorer forces his way up through a narrowing in the rock in a cave high in the White Mountains on the island of Crete.

Cave explorer crawls through a low section of cave passage underground in Derbyshire England.

A cave explorer reaches the top of a waterfall underground in New Britain

A British cave explorer casually admires the highly decorative formations in this particular passage, in Whiterock (cave). Deep