Raufarholsheillir lava tube cave system, Iceland, Polar Regions

Cave system in the Virunga National Park, Rwanda, Africa

Cave system in the Virunga National Park, Rwanda, Africa

Entrance to the Santo Tomas cave complex in the Vinales National Park, UNESCO World Heritage Site, Pinar del Rio, Cuba, West Indies, Caribbean, Central America

Looking down into the Los Peces fresh water cave near Playa Giron, Matanzas Province, Cuba, West Indies, Caribbean, Central America

A technical cave diver using a diver propulsion vehicle (DPV) to dive into the Blue Springs state park cave system near Marianna, Florida.

Technical diver in cave system, Mexico.

Diver enters the cavern system at Chac Mool cenote in the Riviera Maya area of Mexico's Yucatan Peninsula.

A cavern diver ascends into the light filtering into cenote system at Yucatan Peninsula, Mexico.

Two scuba divers in the Garden of Eden cavern part of the cenote system on the Yucatan Peninsula of Mexico.

A cavern diver enters the Taj Mahal cenote system on Mexico's Yucatan Peninsula.

Cavern divers enter cenote system in Mexico's Yucatan Peninsula.

Scuba diver in the Garden of Eden cavern part of the cenote system on the Yucatan Peninsula of Mexico.

Cave dwellings in Pigeon Valley, Uchisar, Cappadocia, Anatolia, Turkey, Asia Minor, Asia

Cave system, Minami Daito, Daito Islands, Japan, Asia

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

Manjanggul longest lava tube system in the world on the island of Jejudo, UNESCO World Heritage Site, South Korea, Asia

Manjanggul longest lava tube system in the world on the island of Jejudo, UNESCO World Heritage Site, South Korea, Asia

Manjanggul longest lava tube system in the world on the island of Jejudo, UNESCO World Heritage Site, South Korea, Asia

A variety of speleothems including stalactites, stalagmites, columns, straws and drapery at Ngilgi Cave, a limestone Karst cave system near Yallingup in the South West, Augusta-Margaret River Shire, Western Australia, Australia, Pacific

Manjanggul longest lava tube system in the world on the island of Jejudo, UNESCO World Heritage Site, South Korea, Asia

Interior of the Baradla-Domica cave system in the Aggtelek National park which stradles the border between Hungary and Slovakia.

Interior of the Baradla-Domica cave system in the Aggtelek National park which stradles the border between Hungary and Slovakia.

Interior of the Baradla-Domica cave system in the Aggtelek National park which stradles the border between Hungary and Slovakia.

Interior of the Baradla-Domica cave system in the Aggtelek National park which stradles the border between Hungary and Slovakia.

Uchisar, Cappadocia, Turkey

Water reflections in the cave Cueva de los Verdes, illuminations of the cave system of a lava tube designed by Cesar Manrique, Lanzarote, Canary Islands, Spain, Europe

Iceberg in the Uummannaq Fjord System. America, North America, Greenland, Denmark

Iceberg in the Uummannaq Fjord System. America, North America, Greenland, Denmark

Iceberg in the Uummannaq Fjord System. America, North America, Greenland, Denmark

Iceberg in the Uummannaq Fjord System. America, North America, Greenland, Denmark

Cave explorer admires pretty Gour Pool at the most remote part of the system in Whiterock (Cave).