Caving

Škocjan: a vast underground river canyon in the Slovenian karst that gave the landform its name.

Poland's best-decorated cave, opened by a quarry blast in 1966 and full of cave bear bones.

The Vercors shaft that was the first cave taken past −1,000 m — a committing vertical descent.

Both an easy show cave and real wild systems, five minutes apart.

River passages and fossil galleries in one of Europe's largest cave systems.

Europe's longest navigable underground river, boated through the rock at La Vall d'Uixó.

The longest and deepest lava tube on earth, running for 65 km under the forests of the Big Island.

The longest known cave system on earth, with more than 680 km of mapped passages under Kentucky.

Deep free-fall pits where Tennessee, Alabama and Georgia meet, the heartland of American vertical caving.

Þríhnúkagígur — lowered 120 m into a magma chamber; Raufarhólshellir as the easy option.