Geologists discover large underground rivers off Galway Bay and the Aran Islands

Large networks of underground rivers stretching out off the west coast of Ireland, hidden beneath the Galway Bay seabed, have been discovered by geologists from the School of Natural Sciences at NUI Galway.

The significant discovery was made by the researchers following conversations with residents from Inishmaan on the Aran Islands. Typically, islands experience fresh water shortages, however, the residents of the Aran Islands said their well of fresh water never ran out. The find was futher confirmed from fishermen who refer to freshwater risings along the south coast of Galway Bay.

NUI Galway’s Earth and Ocean Sciences researchers concluded the water was likely to be flowing for several miles before emerging as 'freshwater rivers' out of the sea bed of Galway Bay. This water flow is caused by rain falling on the land, it runs through the limestone and then goes out under the sea.

Research carried out by geologists at NUI Galway identified that the 'freshwater rivers' are formed by water dissolving the limestone that underlies southern Galway and north Clare. This karstification process slowly created not only the distinctive topography of the Burren, but the conduits reaching out under Galway Bay and the Aran Islands, which could be up to 30 miles long and more than 70 foot wide. Wells intersecting these conduits produce more water than falls on the island as rain, indicating that the water is sourced from the mainland.

Dr Tiernan Henry, lecturer in environmental geology at NUIG said, “We were told about a well at a local hotel on Inishmaan that had an excellent supply of fresh water. The islands can have water problems, they are always short, but this well went deep into the rock and was getting more water than falls on the whole island.”

The pure bedded limestones that form the islands are found throughout north Clare and south and east Galway. These rocks are particularly susceptible to dissolution by rainfall, and time, forming the distinctive drainage patterns of turloughs, swallow holes, sink holes, and sinking rivers. Almost all of the fresh water in these areas flows in discrete zones within the rock discharging in springs found at the coast and in the bays and sea. The means by which these passages and conduits form is well known and well understood, but where they form, and the shapes they take are less easy to predict.

The research was funded by the Griffiths Award and supported by the university’s Ryan Institute for Environmental, Marine, and Energy Research. For more information, visit the School of Natural Sciences at or the School of Earth and Ocean Sciences at


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