Potholers are 20 metres away from connecting 100 miles of limestone tunnels that would link three counties underground for the first time.

Only a couple of months of burrowing remains before a passage below the Pennine fells near Ingleton, in North Yorkshire, connects the huge local system with Lancashire and Cumbria.

A "dry" circuit of the north of England has been an ambition for some 40 years, following previous landmarks such as the opening of a way between Gaping Gill, a cavern large enough to contain St Paul's cathedral, and Ingleborough cave in the Three Peaks area of North Yorkshire.

The county was linked underground with Cumbria in the 1970s when cavers found their way through sediment-blocked passages connecting shafts at Casterton, near Kirkby Lonsdale. Since then, work has continued on a maze of small tunnels and weaknesses.

The current exploration is being led by a caving group known as the Misty Mountain Mud Miners, whose name describes their painstaking attempts to worm through boulder chokes without risking rock falls. The breakthrough is expected to come between Notts Pott and Lost John cave, above the levels where cave-diving becomes necessary to make further progress.

The prospect, which would attract international cavers for a "three-county experience" in Britain's increasingly popular underworld, follows "fantastic work", according to Ian Lawtonl, a member of the survey team. He said: "When all the links are finally established there could be up to 100 miles of tunnels."

Dye in underground streams has long confirmed the inter-county connection and in January a smoke test showed the existence of a dry way through. Another miner, Andy Walsh, said that the joint system would have some 30 different entrances in the three counties.

"Every new place is different, so when you break through to somewhere no one has stepped in or seen before it is unbeatable. You just don't know what is going to be round each corner," he said.

"It's a very exciting time to be a caver in this area. The new areas haven't been uncovered since before the ice age."