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The Cave of Swallows in Mexico is a 400-metre vertical shaft. It's deep enough to engulf the Empire State Building, yet few people even know of its existence. Caves are earth's final frontier and this programme goes where few have been before.

Deer Cave in Borneo is a daytime retreat for five million bats; their droppings support an entire community of creatures. Shine a light on one massive pile of droppings and the whole place shimmers with millions of dung-eating insects.

Caves also harbour some of the most remarkable and bizarre animals on earth - from cave swiftlets who build nests out of just saliva, to the troglodytic animals that never see daylight or ever set foot on the surface. Troglodytes like the Texas cave salamander and Thailand's cave angel have neither eyes nor pigment, and the entire populations of both are found in just a couple of caves.

From Lechuguilla Cave's astonishing six-metre-long crystals, to the extraordinary snottites of Villa Luz, this documentary provides unprecedented access to the hidden world of caves.