Lough Funshinagh

Ireland's Amazing Disappearing Lake

Useful Information

(53.510120, -8.100231)
Open: no restrictions.
Fee: free.
Classification: Karstkarst lake disappearing lake
Light: n/a
Guided tours: self guided
Photography: allowed
Accessibility: yes
Bibliography: Matthew Parkes, Robert Meehan, Sophie Préteseille (2012):
The Geological Heritage of Roscommon. An audit of County Geological Sites in Roscommon.
Geological Survey of Ireland. Unpublished Report. online pdf
As far as we know this information was accurate when it was published (see years in brackets), but may have changed since then.
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22-NOV-1955 Lough Funshinagh disappears completely in moments.
SEP-1996 the last rapid draining of the lake.
2015 begin of floods.


Lough Funshinagh (lake of the ash tree) is a huge lake, in Irish Lough, which has a rather strange behaviour. It sometimes vanishes pretty fast, but reappears later. Quite notable is such an event in Lough Funshinagh, which happened in 1955. Numerous people eyewitnessed the event, and it was even featured in the local newspaper. They wrote:

Nature was in the mood for magic in County Roscommon on Monday when sharp at 12 noon, Lough Funshinagh, with a farewell roar, completely disappeared.

The events must have been pretty fast, it seems fishermen, fish and 200 swans were quite astonished by the lake's disappearance. The fishermen frantically started to throw the stranded fish up as fast as they could. The water disappeared completely into a huge cavity at the foot of the nearby Ardmullen Hills.

Sometimes the lake is called a turlough or karst lake, but those are generally flooding and drying up in a yearly cycle. In wet years they may stay all year, after very dry years they may stay dry even in winter, but in general they are seasonal. This is not the case for Lough Funshinagh which is normally a lake which does not dry up in summer, it's not seasonal. But in some years the water vanishes pretty abrupt, they say in moments, but we guess several minutes or probably half an hour are more accurate. Wikipedia even says two days, but without giving a source for this. And then it is dry for some time until it reappears "overnight". And there is obviously no simple correlation between precipitation and water level. In 2010 the summer drought in Ireland led to fears that the lake would once again disappear overnight, but it did not. Nevertheless, it has disappeared and re-appeared numerous times over the years, often with 20 or more years between two events.

But despite being not a turlough it is something similar and definitely related to karst. The water vanishes in a cave, which must have a decent dimension to swallow such a huge amount of water in short time. The "cave" where the water vanishes is called a swallow hole. The karst water is connected to the lake by caves, and according to the law of communicating vessels the water eve is the same in all connected caves and lakes. So if the water level rises during the rainy winter the lake rises, and whe it goes down because of less rain, the level of the lake goes down. But here we have a different situation: The caves are empty (air filled) and not connected with the lake, then something happens which allows the water to flow into the cave below. Then this connection is blocked again and the lake is refilled by water flowing in on the surface, which is fast but not as fast as the outflow.

What is mostly unknown is the reason for the plug, which normally keeps the water in the lake. One possibility is a layer of impermeable clay. If this seal of clay is broken then the already shallow lake may drain out leaving the lake bed dry. The rivulets bring fresh clay which deposits at the outflow and after some time the outflow is blocked again. Or gradual slumping inward of impermeable material from the sea bed may then reseal the swallow hole. Okay, this theory is ramshackle and there are obviously other strange theories which are also possible. For example, it could be a result of plants, like seaweed or even leaves from trees which flow into the swallow holes and block them. It takes years for the biogene matter to decompose, especially as it is covered by water, but when it is weakened enough the pressure of the lake above flushes it through the cave.

In other words, its obvious that the caves are plugged by an impermeable seal and sometimes the plug is destroyed. But it is unknown how the plug forms, what it is made of, and why it is destroyed sometimes.

Because of its unique situation Lough Funshinagh is considered a Special Area of Conservation in Co. Roscommon. However, geological or speleological research seems to be rather insufficient. The problem is that researchers who do research on this amazing disappearing lake, can never know if and when it will ever happen again. What is known is that the underground is karstified limestone with a lake bed consisting of a thin layer of clay or marl deposits. And it is an extremely rare phenomenon, said to be the only of its kind in Ireland, and actually we are not aware of anything comparable anywhere else. We have not listed any other disappearing lake on showcaves.com so far.

Since winter 2015 the lake became infamous for the opposite of draining. The lake has no outflow on the surface and was drained continually through swallow holes, so it has the same level all year. It seems the climate change caused extra rainfall, which has simply overburdened the swallow holes and now the lake floods its surroundings regularly, causing severe damages. The local government started emergency flood relief works to drain the lake with an overflow pipe to divert the water into the River Shannon. But the lake is a Special Area of Conservation (SAC) and the charitable organisation Friends of the Irish Environment (FOIE) were not happy with the effects on the environment. They sued and the works were stopped by the High Court, much to the dismay of the local inhabitants whose houses and farms were flooded. However, in 2021 the works resumed at a much smaller scale, and in 2022 a much less invasive solution was built. It drains in case of a flood, but does not influence the lake in any other way.