There is an enormous cavern called Gaping Gill on the Yorkshire dales in England. The river drops for 120 metres to the bottom of the cave and then disappears again in another cave called Ingleborough. For over a hundred years geologists have tried to follow the course of this underground river. This film records the first successful crossing made by Geoff Yeadon and Geoff Crossley, accompagnied by the cameraman Sid Perou and the recordist Chris Gibb.