Mossdale Caverns lie on the southern slopes of Great Whernside about 4 kilometres north east of Conistone in Wharfedale and contain over 6 miles (10km) of underground tunnels and passages. They were first explored in 1941 and a return trip to the far end takes between eight and ten hours.
Mossdale Beck disappears into the caverns at the foot of Mossdale Scar and doesn't resurface until Black Keld on the River Wharfe just south of Kettlewell; a distance of 4 kilometres as the crow flies. The linking section of caves between the two has yet to be discovered and is seen as a holy grail for some cavers.
Regarded as a very dangerous cave network, the caverns have a fearsome reputation as they are prone to complete flooding and many of the passages can only be crawled through, while others are neck deep in water.
The system was the scene of Britain's worst caving tragedy in 1967 when six cavers were drowned after the water levels rose following a thunderstorm. The rescue became the biggest ever mounted in Britain, involving over 300 people and led by the Upper Wharfedale Fell Rescue Association and the Cave Rescue Organisation. Eventually the six bodies were found but the coroner decided it was too dangerous to try to get them to the surface and the entrance was sealed. Later the bodies were buried in a chamber in the system that is believed to be above the flood level.