Lag na Sionna
|Location||Derrylahan townland, County Cavan|
|Primary inflows||Shannon Cave|
|Primary outflows||River Shannon|
|Catchment area||12.8 km²|
|Max. length||16 m|
|Max. width||16 m|
|Max. depth||14.6 m|
Shannon Pot or Legnashinna (from Irish: Lag na Sionna, meaning "hollow of the Shannon") is a pool in the karst landscape near Cuilcagh Mountain in County Cavan, Ireland. An aquifer-fed naturally fluctuating pool, it is the traditional source of the River Shannon.
The pool itself is about 16 m (52 ft) wide. It was first explored by divers in 1971 to a depth of 6 m. It was later explored by Martyn Farr among others to a depth of 9.5 m from where the water emerges from a 2 m wide fissure. In early 2009 the pot was dived to a depth of 14.6 m by Artur Kozłowski, who entered an unstable chamber.
According to legend, the Shannon is named after Sionnan, who was the granddaughter of Manannán mac Lir, the god of the sea. She came to this spot to eat the forbidden fruit of the Tree of Knowledge, which was planted by the druids. As she began to eat it, the waters of the pool sprang up and overwhelmed her. She was drawn down into the pool and its water began to flow over the land, forming the River Shannon.
Surveys have defined a 12.8 km2 (4.9 sq mi) immediate catchment area covering the slopes of Cuilcagh. This area includes Garvagh Lough, 2.2 km to the northeast of the pot. Water from Garvagh drains into Pollnaowen[n 1] sink, before emerging at Shannon Pot.
The highest point in the catchment is a spring at Tiltinbane on the western end of the Cuilcagh mountain ridge; this sources an unnamed stream which itself feeds into Shannon Cave. Further sinks that source the pot include Pollboy and, through Shannon Cave, Pollahune in County Cavan and Polltullyard and Tullyrrakeeragh in Fermanagh.
Surveys suggest that Shannon Pot may once have had a much bigger catchment area. In times of high flow it has been shown to be hydrologically linked to Badger Pot and Pigeon Pot beside Florencecourt Forest Park, Fermanagh.
- 'Poll nm1:' hole, pit, sink, leak, aperture (The Pocket Oxford Irish Dictionary – Irish-English)
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- Elmer, Philip; et al. Springs and Bottled Waters of the World. Springer. ISBN 3-540-61841-4.