University of York cavers are treading where no one has walked before. This summer they returned to the mountains of southern Europe to continue exploring a cave system they first encountered several years ago. MATT CLARK reports.

SANDWICHED on the Adriatic, between Bosnia and Albania, Montenegro is a land of vertiginous mountains, wine-dark lakes and stunning beaches. But it's still a relatively unknown country. The new Riviera some would have you believe, with an onslaught of tourists waiting around the corner.

But for members of the University of York cave and pothole club the place to explore is not along the coast, rather it's the Durmitor National Park, a UNESCO World Heritage Site and home to some seriously deep, but, as yet, unexplored caves.

"Its a huge limestone plateau, next to the deepest gorge in Europe," says PhD student Mark Sims, "The vertical range is one and a half kilometres, which meant there are definitely large caves within the mountain. The question was could we find them and if so could we access them?"

To seek answers, several years ago Mark and two fellow cavers set off on a recce to Durmitor and it didn't take long to strike gold. Previous groups had come away empty handed, so they had decided to head north into one of the massif’s most remote areas. There the team happened upon a huge, snow-filled hole, which they christened JVC.

"It was as deep as a five storey building, with ice in the bottom," says Mark. "We could get round the side and there was a tunnel through the ice, which means air is flowing out.

"Finding a good draught is the holy grail; it means there's a big space behind."

Strictly speaking, being vertical this was a pothole, caves are horizontal. But whatever you call them, without any gear, further investigation would have to wait.

So in 2012 an expeditionary force set off from York, and what they found didn't disappoint.

"After negotiating a steep, slippery ice tunnel we followed a passage that twisted through huge walls of ice," says Mark. "Eventually it broke into a large chamber containing beautiful, ancient ice formations."

Beyond this stunning grotto, the team continued to descend into ever colder passages until, frustratingly, they ran out of rope at 150 metres.

Dropping stones and timing how long they took to hit the bottom suggested there was at least another 20 metres to go, then the cave appeared to go round a corner.

How much was left to explore? There was only one thing for it; go back with more people and a lot more rope.

Unfortunately, nature had other ideas. Huge amounts of snow fell in early 2013 and when the group returned in July they found their cave totally sealed off.

Luckily another entrance had been spotted on the first recce. Named Bunda Jama, it was discovered by negotiating a 40 metre shaft into a huge boulder floored chamber, big enough to house several double-decker buses stacked on top of each other.

"Beyond, there were small passages that required days of digging before we gained access to the deepest part of the cave," says Mark. "The biggest shaft we found was, at 68 metres, one third as high again as the central tower of York Minster. We followed a howling draft to minus 300 metres."

But, despite having taken double the amount of rope, again it was not enough.

Which meant this August's expedition would be a double mission and to be on the safe side the team took with them more than a kilometre of nylon braiding.

At least the weather Gods were smiling, because this time round JVC was wide open.

"We got to the last bolt in the wall from the previous trip (at 150 metres), started a new rope and abseiled down an ice slope that dropped off into an enticing black space,” says Mark. “There was just nothing below.

"We abseiled through some cold water dripping from above, then the floor was ice again."

Astonishingly what previously seemed to be the floor was no more than a huge sheet of ice. Then another big shaft. Over the course of the three week expedition, the cavers had pushed down to 250 metres.

From the second cave entrance, Bunda Jama, they went even deeper.

"It just carried on and on. By the end of the trip we had pushed down to 622 metres below the surface."

That alone makes it the fourth deepest cave in Montenegro and twice as deep as the deepest in Britain. Indeed, towards the end each exploratory trip was taking more than 20 hours to complete.

And it wasn't a lack of rope that brought proceedings to a close.

"At 622 metres we found a long lake where the roof comes down and meets water. The cave hasn't finished but the passages are flooded. So that's as far as we can go as dry cavers."

"However water from the surface emerges again at the bottom, and that means it would be highly unlikely that everything below is flooded."

If so, it could be one of the deepest caves in the world. Mark says there's talk of going back in 2016 with a couple of divers to see what's beyond, but there are plenty of other challenges out there.

So what is it that draws him into the subterranean time after time?

"An astronaut once told a cave explorer that they were the only ones who go on real expeditions and I know what he meant. If you climb a new peak, others will have seen it in photos. You can even go on Google Earth to see what the view will be like.

"Everything's been mapped, it doesn't matter how remote. But when you go on a caving expedition, you can't know until you see it.

"The last real wilderness is underground."