Basilica Cistern with an orange illumination, Istanbul, Turkey, Europe

Basilica Cistern with an orange illumination. Istanbul, Turkey, Europe

Basilica Cistern with an orange illumination, Istanbul, Turkey, Europe

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

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

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.

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

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