The biggest underground chamber has been discovered beneath the Mendip Hills.
Now named The Frozen Deep, it took six people nicknamed the Tuesday Diggers four years to tunnel to it under Cheddar Gorge.
The 1,800sq m cavern is not open to the public but the first images taken there reveal 5m tall calcite columns and white flowstone covering the walls and floor.
Hugh Cornwell, director of Cheddar Caves and Gorge, said: "The question, already emerging, is 'Can the diggers and the divers find a connection from The Frozen Deep into the River Cave to continue the exploration?"
The River Cave meanders under the gorge and cannot be accessed further due to being boulder choked. The underwater pass is too tight to pass with air tanks and too narrow to escape after setting a charge.
Mr Cornwell said: "For the moment, however, cavers across Mendip will celebrate the courage, endurance and spectacular achievements of the diggers. The diggers will consolidate their find by taping out walkways to protect the calcite formations from damage and carry out laser surveys of all the chambers, before allowing a limited number of cavers under their supervision to visit The Frozen Deep."
The breakthrough came on Tuesday last week and the discoverers are Martin Grass, Alison Moody, retired GPs Dr Pete Glanvill and Dr Tony Boycott, Nigel Cox and Nick Chipchase, who had his 65th birthday the day after discovery.
The six of them have had exclusive access for the last four years from Longleat Estate to dig the gated Reservoir Hole, which Willie Stanton, as Lord Bath's consultant geologist, had previously looked after for Longleat Estate for many years. It lies 150m east of The Pinnacles in Cheddar Gorge and has a large main rift, which was discovered and named Golgotha by Mr Stanton in 1969. Further major finds were made in 1970 and 1973, but progress thereafter was more limited.
Mr Stanton, who died in 2010 had suggested that the diggers concentrate on exploring a side passage, and eventually they broke through into a 20m long parallel rift, which they named Great Expectations. Removing a large slab enabled them to crawl a further 15m into another chamber, 25m high and 20m long, which they named Resurrection Chamber. This led them to a loose boulder slope on September 3, which ended in a 12m vertical pitch. They returned the next day with rope and tackle and descended the pitch into the largest chamber yet discovered under the Mendip Hills.