Special milestone for underground caves
When famous cave explorer Edouard Alfred Martel discovered the Marble Arch caves in Fermanagh in 1895, he thought they would make a good tourist attraction.
Ninety years later, the caves were opened to the public, and Martel was proved right.
About 1.2m people have descended the steps deep below the Fermanagh countryside to explore a subterranean world of rivers, waterfalls, winding passages and high chambers.
The age of the caves is not known, but they are thought to have been formed over 380,000 years ago.
Friday marks the 25th anniversary of their opening in 1985.
Back then, the BBC filmed the St Macartin's Cathedral Choir from Enniskillen, among them the young chorister Neil Hannon, who went on to achieve fame with his band, the Divine Comedy.
Now the caves are celebrating their own success story.
"Leap of faith"
They have become one of Northern Ireland's most important tourist destinations.
Walkways and electrically powered boats allow visitors to see an array of cave features, including stalactites, stalagmites, fragile mineral veils, cave curtains and rimstone pools.
The exceptional geological heritage of the caves has been recognised and in 2004 the area was awarded Global Geopark status by UNESCO.
Richard Watson, manager of Marble Arch caves has been working at the caves since they opened.
He told BBC Radio Ulster that no-one imagined just how successful a tourist attraction they would be.
"It was a great leap of faith by Fermanagh District Council at the time to have the vision to develop a tourist attraction of this scale and extent," he said.
He pointed out that 1985 was "a troubled time in NI's history" just two years before the Enniskillen bomb.
"Things have moved on a lot and I think we have made our mark as a tourist attraction but I don't think anyone could have foreseen just how successful we've been," he said.
In the early days, he found himself spending a lot of time underground.
"The first season, because we were so unexpectedly busy, it was just all hands to the pump really," he said.
He spent six weeks without a day off, and after noticing blotches on his skin was diagnosed with vitamin D deficiency.
"I was falling apart through lack of sunlight," he said.
Mr Watson said any changes to the cave formations over the past 25 years would be "imperceptible" in human terms.
"The fastest growing stalactite down here has maybe grown by a tenth of a millimetre. We're talking about microns," he said.
The rock formations will also have changed little since Edouard Martel descended into the caves over one hundred years ago, but 25 years on from the public opening, the caves may have become the tourist attraction he imagined.
The Marble Arch Caves are open daily until the end of September 2010.