A lake in Northern Ireland repeatedly appears, floods the road and drains away within hours; no one has fully mapped where the water goes

The majority of landscapes act as common sense dictates. Lakes remain where they are, rivers flow downward towards the sea, and roads built over arid land typically stay roads rather than become open water. However, a small upland lake tucked away in Northern Ireland's County Antrim hills appears to have completely missed the memo, filling and emptying at a rate that has perplexed both residents and geologists for generations. Although the lake is known as Loughareema, practically everyone in the vicinity just refers to it as the Vanishing Lake. Depending on the day, a driver may see a large sheet of dark water lapping at the tarmac on a public road that passes directly across the centre of the basin.Water flows in, but there is no way outThree distinct streams flow into Loughareema from the surrounding hills, yet not a single stream has water flowing out on the surface. Instead, the lake drains through a sinkhole at its base into an underground drainage system, with the water re-emerging about 2.5 kilometres away at a spring in the Carey River. The destination of the water is known. The detailed underground drainage system has not yet been fully characterised. Geological mapping and tracer testing have linked the sinkhole to a spring in the Carey River about 2.5 kilometres away, while the exact network of channels and faults between the two remains more difficult to trace.The way the drain works is what makes the lake so unpredictable. Water flowing into the basin brings in loose peat and debris. Dr Paul Wilson, a hydrogeologist with the British Geological Survey, has been a driving force behind recent efforts to adequately understand the mechanism. He has installed time-lapse cameras and water level loggers around the site to record exactly when and how fast the lake changes condition.The underground drainage system has never been properly surveyed, even with modern instruments focused on the location, partly because there is no safe or evident way to follow a channel that flows invisibly through solid rock. Wilson described the lake as a place where visitors could never be certain what they would find, because its water level could range from completely empty to full depending on recent rainfall.Ghost stories built on a very real geological puzzleLong before hydrogeologists took an interest in Loughareema, local storytellers had already built up a whole folklore about its capricious moods. The most famous legend is a horse-drawn carriage that attempted to cross the road through the lake one night in the 19th century, but the water rose unexpectedly and drowned the horses, the carriage and its passengers. A local legend says that on nights when the lake is full, the ghost of the carriage can still be seen along the shoreline.The site's scientific value has also been formally recognised well beyond folk tales. National Museums Northern Ireland claims that ephemeral lakes with this kind of quick self-regulation of drainage are so unusual anywhere in the world that Loughareema is included in its Earth Science Conservation Review as a site of national geological importance.The riddle is, for the moment, just half unravelled. Scientists know enough about the general mechanism to explain why the lake disappears, but the finer details, such as how old the drainage system is and how far its channels go underground, are still being worked out. Until that mapping is done, anyone driving the winding road through Ballyvennaght can only do what generations of visitors before them have done: look ahead and wonder if today is a day when the Vanishing Lake will live up to its name.Catch the latest World News and Live updates. Download the TOI app.