Somerset cavers spend their retirement tunnelling into the Mendips
To uncover the secrets of what is thought to be the largest cavern ever found in the Mendip Hills in Somerset has taken a group of cavers more than four years of dedicated tunnelling.
And to get to the vast cavern takes half an hour of grovelling on hands and knees, scrambling across massive boulders and squeezing through 700 ft of tiny passages.
So why would a group of retired doctors, teachers and engineers spend their retirement buried underground tunnelling deep into the hills of the Mendips?
"Nobody asks Chris Bonnington why he has to go up Everest when you can see the top of it," says Peter Glanvill, a retired GP from Chard and one of the cavers.
"With caving you don't know what's around the next corner - you're going into the unknown.
"And where else can you go where'll you be the first person ever to tread."
Last Tuesday the group of cavers, known as the Tuesday Diggers, became the first people ever to step into a 30m (98ft) high chamber known as The Frozen Deep.
It had taken the group, whose average age is 60, up to four hours a week since 2008 to make the discovery.
"We're like 'Last of the Summer Wine goes caving'," laughs Mr Glanvill.
"But in the Mendips it's all about patient excavation and hard graft - you can't just stroll up, shift a boulder and jump down into a cave system - you have to keep the faith and keep slogging.
"That's why more mature cavers can keep it up - they're less impatient and can dig for years in the hopes of discovering something."
And they are not the only group busily digging exploratory tunnels deep into the hills around Cheddar Gorge.
There are thought to be at least seven "active digs" in the Mendips and the oldest, Templeton, has been active for the past 15 years.'Rugged individuals'
And there are, according to Mr Glanvill, advantages of spending years digging your own tunnel system into the hills.
"I'm 6ft tall and weigh about 14 stone - and I can get through," said Mr Glanvill.
- Cheddar is Britain's largest gorge, boasting an elaborate cave system and limestone cliffs which rise from the valley floor
- Britain's oldest complete skeleton - the "Cheddar Man" - was found there in 1903, having been buried below ground for 10,000 years
- Cave systems exist all over Britain, with the longest a 70 mile (113km) network linking Cumbria, Lancashire and Yorkshire
- One of the world's most spectacular caves is considered to be Mexico's Cueva de los Cristales (cave of crystals)
"Digging a tunnel you can make it big enough to get through - if it was too tight we enlarged it to make sure we could all get through."
But despite customising your tunnel system, how does someone squeezed into a dark narrow shaft, deep underground, overcome their claustrophobia.
"It is a relative thing, it just depends on how much you're willing to put up with and as you get older you can't tolerate as much," admits Mr Glanvill.
"There were a number of occasions when I've been quite worried that boulders might fall.
"And some of the group reported hearing rumbles from beyond in the cave - which was inspiring and alarming.
"But you don't want to have too overactive an imagination."
Ultimately though, according to Mr Glanvill, the draw of caving is not just original exploration but the similarities with his boyhood hero Professor Challenge.
The fictitious character created by Sir Arthur Conan Doyle featured in the author's 1912 novel The Lost World.
"Cavers are rugged individuals, they all have beards," joked Mr Glanvill.
"I've got one and I've read all those adventure stories and really I just wanted to be like Professor Challenge."