Shannon Cave

Shannon Cave is an active stream passage cave which straddles the border of County Fermanagh, Northern Ireland and County Cavan in the Republic of Ireland. At 130 metres (430 ft) vertical depth, it stands joint sixth with Poulnagree in County Clare in the deepest caves on the island of Ireland. The cave is recorded at 5.4 kilometres (3.4 mi) in length (making it the sixth-longest on the island of Ireland), but exploration is ongoing and further passage is expected to be found.

Cave data from Wikidata (CC0). Summary from Wikipedia (CC BY-SA).