Vincent Grafhorst / Minden / River in sandstone cave, Sof Omar Cave, Ethiopia

Vincent Grafhorst / Stalagmites and stalactites in cave chamber, Gcwihaba Caves, Botswana

Michel Roggo / Karst spring where water bubbles up from subterranean source, Julian Alps, Slovenia, June 2008

Solvin Zankl / European Mole (Talpa europaea) catching worm in its subterranean burrow, Germany. Live, captive animal.

Solvin Zankl / European Mole (Talpa europaea) catching worm in its subterranean burrow, Germany. Live, Captive animal.

Solvin Zankl / European Mole (Talpa europaea) in its subterranean burrow. Germany. Live, captive animal.

Solvin Zankl / European Mole (Talpa europaea) in its subterranean burrow, Germany. Live, captive animal.

Mitsuhiko Imamori / Honeypot Ant (Camponotus inflatus) repletes hanging from ceiling of larder, engorged with nectar they will regurgitate on demand to other workers, central Australia

Heather Angel / Blind subterranean fish (Sinocyclocheilus sp) with head bump and no eyes. Found in Jiuxiang Cave. Captive in aquarium, Kunming, Yunnan, China.

Duncan Murrell / Mangrove snake (Boiga dendrophila) in the Puerto Princesa Subterranean River National Park, UNESCO World Heritage Site in Palawan, the Philipines.

Duncan Murrell / Palawan swiftlets (Aerodramus palawanensis) returning to the subterranean river cave, Puerto Princesa Subterranean River National Park UNESCO World Heritage Site, Palawan, the Philippines. Endemic to...

Duncan Murrell / The buttress roots of a large tree in the Puerto Princesa Subterranean River National Park, UNESCO World Heritage Site in Palawan.