Cascades of mammoth ferns flourish in the humid air trapped between the narrow walls of Claustral Canyon. First explored in 1963, the formation was named for its claustrophobia-inducing passages and ranks among the regions most visited canyons. (Photograph by Carsten Peter)
These are the impressive Škocjan Caves in Slovenia , where the Reka River disappears deep underground. People have lived in the caves since prehistoric times, and the internal atmosphere even has its own eco-system.