There is a 'Pool of Tranquility' hidden beneath Donegal's Owey Island, as this breathtaking footage reveals.
The video, captured by local adventurer Iain Miller of Unique Ascent (uniqueascent.ie), reveals an extraordinary 'lake beneath a lake' on Owey.
It is believe to be the first footage ever shared publicly from the cave.
Owey Island is located some 500m beyond the northern tip of Cruit Island, off the coast of western Donegal. It is inhabited only during summer.
A small lake can be found on the island, and beneath it is a 150m-long underground cavern accessed via a narrow sinkhole. The cavern contains a second, subterranean lake, fed both from above and from the sea at a collapsed cave entrance.
The underground lake ends in an eight-metre pool that has been dubbed the 'Pool of Tranquility' due to its haunting darkness and silence.
"I sat in darkness in the dingy in the Pool for eight very long minutes, the only person on the island," Miller told Independent.ie Travel of a recent visit.
"The cave/cavern is huge and the darkness is almost overwhelming when combined with the silence and solitude. Your brain and eyes have no reference point and sensory deprivation quickly forces the brain to melt," he added.
Miller runs rock-climbing and adventure tours of Donegal, but has made a name for himself thanks to daredevil coastal exploits - including the first ascent of Mayo's iconic Dún Briste sea stack in 25 years last summer.
As with his Downpatrick Head climb, accessing the Owey Island cave is an extremely hazardous activity, and and should not be undertaken without suitable expertise, safety equipment or fully qualified guides.
"This is not a place to venture without a shed-load of artificial light and underground/steep ground knowledge," Miller says.
The Pool of Tranquility was first visited in 2009, he adds, when he and his son Oisín joined a local caving couple from Belfast - Stephen 'Jock' Read and Emma Ross - to reach it using an inflatable dingy.
"The temperature underground is around five degrees centigrade all year round," he says. "It feels warm in winter and freezing cold in summer."
NB: The activities described in this article were undertaken by a pro at his own risk. Caving can be extremely dangerous, and should never be undertaken without suitable levels of expertise or fully qualified guides.