Cavers set to connect Yorkshire, Lancashire and Cumbria
Cavers say they are just metres away from connecting Cumbria, Lancashire and Yorkshire through underground passages.
The team has been working for about 20 years to connect a 60 mile network of limestone tunnels.
The believe they are now just 20m away from linking a passage below the Pennine fells near Ingleton, in North Yorkshire, with Lancashire and Cumbria.
They are expecting to break through near to Lost John's cave in North Yorkshire.
Tim Allen and Hugh St.Lawrence, from Ingleton, are part of a team of enthusiasts known as the Misty Mountain Mud Miners.
The team of friends have been working in their spare time to connect the tunnels, and usually go underground for a few hours once a week.
Mr Allen said: "Sometimes we move along a metre or two in a session, sometimes we make little progress for a month."
He added that the connection would only be complete once they were able to move through the remaining boulders.
A sound test has confirmed the two ends are quite close together, although the difficulties remain considerable.
Mr St. Lawrence said: "The system should be 100 kms (62 miles) long when connected and may move up into the top 20 longest caves in the world - not bad considering the tiny amount of limestone we have in this country compared to some!"
He added that they could not put a time frame on exactly when they might eventually connect the counties.
He said: "It could be two months, six years, or never. When you're going into the unknown, and the unknown is a giant pile of thousands of tons of loose rock, it's impossible to say how quickly it will go.
"We just have to chip away at it and hope for the best."