Beneath the familiar landscapes of our planet lies a hidden world of sprawling labyrinths. For millions of years, water has dissolved layers of stone to create massive underground networks of caves and tunnels that stretch for hundreds of miles.
These subterranean giants are delicate ecosystems and time capsules. They have sheltered unique organisms and human communities for millennia, and are full of stories and wonder.
This gallery explores the most extensive cave systems currently known, ranked by their length. Click through to explore them.