To the south of Lower Lough Erne lies a limestone plateau, where Fermanagh's abundant rainwater has carved out a network of subterranean caverns. The largest of these are the star attraction of the Marble Arch Caves Global Geopark. Popular 1¼-hour tours feature spectacular chambers and underwater rivers.
The caves are 16km southwest of Enniskillen, and 4km west of Florence Court, reached via the A4 and the A32. Book tours in advance in summer.
Tours begin with a short boat trip along the peaty, foam-flecked waters of the underground River Cladagh to Junction Jetty, where three subterranean streams meet up. You then continue on foot past the Pool Chamber, regaled all the time with jokes from your guide. An artificial tunnel leads into the New Chamber, from which the route follows the underground River Owenbrean through the Moses Walk (a walled pathway sunk waist-deep into the river) to the Calcite Cradle, where the most picturesque formations are found.
Unexpected serious flooding of the caves in the 1990s was found to have been caused by mechanised peat-cutting in the blanket bog on the slopes of Cuilcagh Mountain, whose rivers feed the caves. Cuilcagh Mountain Park was established to restore and preserve the bog environment, and in 2001 the entire area was designated a Unesco geopark. The park's geology and ecology are explained in the caves' visitor centre.
The caves take their name from a natural limestone arch that spans the River Cladagh where it emerges from the caves; you can reach it via a short walk along a signposted footpath from the visitor centre. They were first explored by the French caving pioneer Édouard Martel in 1895.