A lake-burst (Old Irish: tomaidm, Irish: tomhaidhm) is a phenomenon referred to in Irish mythology, in which a previously non-existent lake comes into being, often when a grave is being dug. Part of the lake-burst stories may originate in sudden hydrographic changes around limestone-based inland plains or turloughs. Other so-called lake-bursts refer to marine estuaries, bays and inlets, such as Galway Bay, Strangford Lough, Dundrum Bay, Belfast Lough, Waterford Harbour and the mouth of the River Erne. Some of these coastal districts were renowned for the drowned prehistoric forests, which gave rise to several flood-myths.
Medieval bards had a special genre of lake-burst poems called tomamond. More or less elaborate 11th- or 12th-century narratives have survived around Galway Bay, Lough Neagh and Lough Ree, which seem to be related to similar (though less ancient) stories in Wales (Cantre'r Gwaelod, Llys Helig, Bala Lake, Llynclys), Cornwall (Lyonesse), Brittany (Ys) and Normandy (Forêt de Scissy). A late 16th-century Frisian legend, probably borrowed from Irish examples, refers to the origins of the Zuiderzee. Other Irish texts refer to the eruption of the River Boyne and other rivers. The poems of the lake-burst of Lough Erne and the eruption of Brí (where the legendary character Midir lived) have been lost. In Wales the flood-myth is elaborated in the story of Dwyfan and Dwyfach, who saved people and animals from the great deluge caused by the monster Avanc living in Llyn Llion (possibly Bala Lake). Its Irish counterpart as told in the Lebor Gabála Érenn only links up with the Biblical story of Noah's flood.
The theme relates to the classical story of the warrior Marcus Curtius, who was said to have thrown himself in the Lacus Curtius near the Forum Romanum in order to stop a chasm made by the river Tiber. A similar story was told about King Midas.
Not every lake mentioned in medieval sources can be identified with certainty. Loch Lainglinne, for instance, might be another reference to Belfast Lough, which was known as Loch Laoigh or Loch Laigh.
Apparently, medieval Irishmen were convinced that almost all of their lakes had emerged after Noah's flood. Their myths suggest that land reclamation and deforestation went hand in hand with the seasonal inundation of low-lying plains. According to the corrupted text of Lebor Gabála Érenn king Partholón found only three lakes or bays: Loch Fordremain in Sliab Mis of Mumhan (Tralee Bay), Loch Lumnig (probably Loch Lurgan or Galway Bay) on Tir Find and Loch Cera or Findloch over the borders of Irrus. Interestingly enough, several major lakes and outlets, such as Lough Corrib, Lough Derg (Shannon), Shannon Estuary and Killary Harbour are not mentioned in any known myth. This may imply that at least some lake-bursts mentioned have been wrongly identified with smaller lakes, where, in fact, they were referring to one of the major lakes. Lough Derg (Ulster), moreover, was identified in the Life of Saint Patrick as the location of Purgatory.
The plains that supposedly had been drowned, had special names, which have been preserved in a 16th-century manuscript.
List of mythical lake-bursts
Fionn mac Cumhaill's time
- Loch Laighlinne in Ui mac Uais of Brega (Laighlinne's grave)
- Loch Rudraige, in Ulaid (Rudraige's grave)
- Loch Techet, in Connachta
- Loch Mese, in Connachta
- Loch Con, in Connachta
- Loch Echtra, in Airgialla (between Sliabh Modharn and Sliabh Fuaid), "full of swans"
- Loch Cuan in Ulaid, an inundation of the sea over the land of Brena, or by the river Brena - the seventh lake eruption
- Loch Annind (Annind's grave)
- Loch Cal in Ui Niallain
- Loch Munremair in Luigne
- Loch Dairbrech (another king's grave)
- Loch Cimbe
- Loch Buadhaigh
- Loch Baadh
- Loch Ren
- Loch Finnhaighe
- Loch Greine
- Loch Riach
- Loch Chaech, in Leinster
- Loch Laegh, in Ulster
- Loch Uair, in Meath
- Loch n-Iairn
- Loch Ce, in Connaught
- Loch Saileann
- Loch nAilleann, in Connaught
- Loch Feabhail
- Loch Gabhair
- Loch Dabhall, in Oirghialla.
Óengus Olmucaid's time
- Aenbheithe, in Ui Cremhthainn
- Loch Saileach
- Loch Na nGasan, in Magh Luirg, in Connaught
- The eruption of the sea between Eabha and Ros Cette
Óengus the Mac Oc's time
- Electronic Dictionary of the Irish Language, s.v. tomaidm.
- Foclóir Gaeilge-Béarla, s.v. tomhaidhm.
- Plutarch, Moralia: Greek and Roman Parallel Stories
- Most identifications after John O'Donovan (ed.), Annals of the Kingdom of Ireland, by the Four Masters, from the Earliest Period to the Year 1616, 2nd. ed., vol. 1, Dublin 1856.
- John Carey, 'The names of the plains beneath the lakes of Ireland', in: John Carey, Máire Herbert, Kevin Murray (eds.)Cín chille cúile: Texts, Saints and Places: Essays in Honour of Pádraig Ó Riain, Aberystwyth 2004, p. 44–57.
- Harry Roe, Ann Doole (eds.), Acallam Na Senórach, Oxford 1999, p. 127-128.
- "Lebor Gabala Erenn pt. 2". Retrieved 16 January 2011.
- Identification after Hector Munro Chadwic, Early Scotland: The Picts, the Scots and the Welsh of Southern Scotland, Cambridge 1949, repr. 2013, p. 107. Gerardus Cambrensis calls it Lake Ruturugus.
- "Lebor Gabala pt. 3". Retrieved 16 January 2011.
- CELT: The Corpus of Electronic Texts: Annals of the Four Masters