A group of potholers stopped from exploring because of the foot-and-mouth epidemic, found a network of caves under the car park of their local pub.
To stave off boredom, members of the Bristol Exploration Club volunteered to help clear out a drain at the Hunters Lodge Inn at Priddy in Somerset.
But instead of finding a blocked pipe, the group stumbled on a network of previously unexplored caves.
After two years digging and blasting, they have now opened up a 30-ft cavern.
Inside they found hundreds of bones of extinct animals - believed to have been washed into the cave nearly 10,000 years ago - and an underground world of stalagmites and stalactites.
Tony Jarrett, 54, team leader of the group which is based in the village, said: "We have been digging for years in the area trying to discover new caves and expand previously discovered ones.
"There was a two-inch natural fissure in the rock into which the rainwater from the pub roof and the car park used to drain.
"We suspected there was something down there as the water had to escape somewhere.
"So we went down and popped out into a cave of stalactites and stalagmites - we were amazed."
The cavers have named the caverns the Pewter Pot, the Barmaids' Bedrooms and
Brown Ale Boulevard, in honour of the Hunters Lodge.
Experts at the British Museum have identified the discovered bones as belonging to animals which roamed Britain during the last Ice Age - many of the finds are on display at the nearby Wells Museum in Somerset.
The Mendips are home to some of Britain's best-known caves, including Wookey Hole.