It has remained out of sight for an unknown length of time and has aquatic species living within that are believed to never have seen daylight.

But two Scots have discovered what is believed to be the country’s longest cave after coming across the nettle-choked entrance by chance as they explored the area.

Geologist Iain Greig of Aberdeen and Neal Menzies from Stonehaven were exploring an area about two miles south of Durness in Sutherland last June.

Around 500 yards from the road in the North West Highlands Geopark, a geological area which is home to a number of limestone caves, they stumbled across an innocuous looking entrance which was blocked by vegetation.

The experienced cavers believe that it could directly to the world-renowned Smoo Cave near the village which is a distance of around four miles away.

When the men entered what has now been called the Cave of the Black Stones, they found the way blocked by silt but it was heading northwards towards Smoo Cave, following the course of River Dionard.

Experts have long believed the thin line limestone rock which is flanked on either side by harder rocks has had the potential to be full of cave formations as surface water drains straight through the soil even in the heaviest rain.

Now a team of cavers is to mount an expedition to drill through the silt which will allow them to explore the system in full.

But during the initial visit they discovered a freshwater eel which they believe has never been exposed to sunlight.

Mr Greig, 34, said: “We were not expecting what we found. It was full of nettles and less than a metre wide. We managed to enter it and there was a section of boulders – we moved them out of the way and then there was a 17ft drop. That led to a spectacular chamber 15 yards by 16 feet – big enough to stand up in. The stones were blue and purple. It was some incredible sight.

“We managed to explore 150 yards before finding the route choked with sediment. We plan to return with a team in the New Year to dig it out and explore further.

“The theory and hope is it could extend all the way to Smoo Cave which would make it by far the longest cave in Scotland and with the most spectacular of entrances – or exits, depending on the start point – at Smoo.”

Scotland’s longest cave is currently the Cave of the Sloping Rock in nearby Assynt which is 1.782 miles long.

The longest cave system in the UK is the Three Counties System in the Yorkshire Dales, with 53.9 miles of passageway, while the longest in the world is the appropriately-named Mammonth Cave in Brownsville, Kentucky with more than 400 miles explored.

Smoo Cave is the largest coastline cave in the British Isles and has provided shelter for thousands of years. It is believed to have been a Stone Age home more than 5,000 years ago with Norse settlers later gathering here to repair boats and fish for herring.

It has one of the largest entrances to any sea cave in Britain at 50 ft high. It was formed by a burn that runs down into the rear chamber, as well as erosion caused by the sea.

Mr Greig added: “If it links up to the new discovery it will be one of the best caving experiences in the UK. It is a treasure trove for geologists. Over the two days we have explored the new cave we even found a white freshwater eel which has never seen sunlight.

“The drought helped us because the cave floods in wet conditions, and up to the roof in places. It’s like a big treasure hunt, but we do not know what the treasure is yet. It is what most cavers dream of. This is the final frontier of exploration in Scotland. Every inch of the country has been mapped by satellite – it’s what remains under the surface that remains the big mystery.”

Fellow caver Colin Coventry, who for more than 30 years has led tours at Smoo Cave, added: “For many years I have been convinced that Smoo was linked to one long cave. We believe the biggest cave in Scotland is here and waiting to be found.

“This looks like it. But it is spectacularly dangerous.

“I would certainly not take tours there. However this is one of the most exciting finds for many years.”