Planeta Tierra es una aclamada serie de documentales creada por la BBC que nos muestra, en 11 episodios escritos por el prestigioso naturalista David Attenborough, la belleza de nuestro enorme planeta con imágenes nunca antes vistas. 4. "Caves" UK broadcast 26 March 2006, 8.98 million viewers (33% audience share);[25] US broadcast 22 April 2007 This episode explores "Planet Earth's final frontier": caves. At a depth of 400 metres (1,300 ft), Mexico's Cave of Swallows is Earth's deepest pit cave freefall drop, allowing entry by BASE jumpers. Its volume could contain New York City's Empire State Building. Equally as impressive, we explore the otherworldly cenotes of the Yucatán Peninsula. Divers appeared to be flying in water as clear as in air, as they give us a glimpse of the hundreds of kilometers of these caves which have already been mapped. Also featured is Borneo's Deer Cave and Gomantong Cave. Inhabitants of the former include three million wrinkle-lipped bats, which have deposited guano on to an enormous mound. In Gomantong Cave, guano is many metres high and is blanketed with hundreds of thousands of cockroaches and other invertebrates. Also depicted are eyeless, subterranean creatures, such as the Texas blind salamander and ("bizarrely") a species of crab. Carlsbad Caverns National Park is featured with its calcite formations. 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, has adapted to this habitat. The programme ends in New Mexico's Lechuguilla Cave (discovered in 1986) where sulphuric acid has produced unusually ornate, gypsum crystal formations. Planet Earth Diaries reveals how a camera team spent a month among the cockroaches on the guano mound in Gomantong Cave and describes the logistics required to photograph Lechuguilla. Permission for the latter took two years and local authorities are unlikely to allow another visit.