This episode explores "planet earth's final frontier": the world of caves. At a depth of 400 metres, Mexico's Cave of Swallows is Earth's deepest pit cave freefall drop, allowing entry by skydivers. Its volume could contain New York City's Empire State Building. Mexico's Cueva de Villa Luz is also featured, with its flowing stream of sulphuric acid and snottite formations made of living bacteria. A fish species, the Shortfin Molly (Poecilia mexicana), has adapted to this habitat.