S1, E4: The Cave of Swallows in Mexico is a 400m vertical shaft, deep enough to engulf the Empire State Building. The Lechuguilla cave system in the USA is 193km long with astonishing crystal formations. Caves are remarkable habitats with equally bizarre wildlife. Cave angel fish cling to the walls behind waterfalls with microscopic hooks on their fins. Cave swiftlets navigate by echo-location and build nests out of saliva. The Texas cave salamander has neither eyes nor pigment. Planet Earth gets unique access to a hidden world of stalactites, stalagmites, snotites and troglodytes.