The Bizarre Lake That Disappears Overnight: Loughareema

The Lake That Vanishes: Unravelling Northern Ireland’s “Vanishing Lake”

On a mist-laden plateau in County Antrim, in Ireland, a lake plays a fascinating disappearing act. One day, motorists along the A2 road might pass a broad expanse of water lapping at the roadside; a few days later, they’ll find only a barren stretch of cracked mud. This is Loughareema – meaning “the lake that runs out.” It’s better known as the Vanishing Lake, an ethereal body of water that fills and drains with bewildering speed. Locals swap stories of ghostly carriages and drowned horses on its shores, but the real marvel here is geological. Loughareema offers a rare window into the hidden workings of the earth, where peculiar rocks and underground waterways conspire to make a lake vanish right before our eyes.

A Karst Landscape Beneath the Bog

To the naked eye, the site of Loughareema looks unassuming – a shallow basin ringed by gentle hills of heather and gorse. Often a blanket of boggy peat and fog hangs low, giving the area an otherworldly atmosphere. Beneath the soggy peat, however, lies the key to the lake’s disappearing trick: limestone, or more precisely the Ulster White Limestone Formation. This thick layer of pale chalky rock, deposited around 100 million years ago in the Cretaceous Period, forms a “leaky” foundation under the lakebed. Limestone is a soluble rock – slightly acidic rain and groundwater can eat away at it, carving channels, cavities, and caves over thousands of years. The terrain that results is called karst, a landscape of hidden drainage where surface streams can vanish into sinkholes and underground rivers. Loughareema sits atop such a karst labyrinth. It’s essentially a shallow bowl with a porous, hole-riddled bottom.

Complicating the geology further, just a stone’s throw from the lake’s centre, the bedrock abruptly changes. The white limestone is fringed by black basalt – hard volcanic rock from long-ago lava flows that once covered this land. Basalt doesn’t dissolve; it’s more like waterproof concrete. Where basalt caps the ground, rainwater can’t easily seep down, so it rushes off the slopes in streams. Over centuries, those streams have converged on the little limestone basin of Loughareema. In effect, nature set up the perfect trick: three streams of water pouring into a hollow with a hidden drain at the bottom. Think of a bathtub – surrounded by solid walls, with an open plughole in the floor. Now imagine that someone keeps pulling and replacing the plug at unpredictable intervals. That, in essence, is Loughareema.

*Image shows Loughareema when full.

The Disappearing Act: How Water Fills and Flees

Normally, a lake has an outlet – a river or overflow to carry away excess water. Loughareema has none on the surface. Its only drain is underground, a vertical sinkhole in the limestone lake bed. Most days, this “plughole” is clogged. Over years, peat from the surrounding bog has washed in and choked the limestone fissures with a natural mulch. While the drain is plugged, any rainwater that flows in is temporarily trapped. The lake starts to rise. After heavy rains, the three feeder streams gush brown, peaty water down the hills and into the bowl faster than it can seep out. The water level climbs, spreading into a shining lake that can expand to cover the valley floor. In Victorian times, a road built on a low causeway across this valley was often submerged without warning; today the road is raised a bit higher, but a particularly deep filling still turns it into a causeway through open water. For a short while, Loughareema becomes a true lake – sometimes only inches deep, sometimes several feet – seemingly out of nowhere.

Then comes the vanishing. As the lake reaches higher levels, pressure builds on that plugged sinkhole. The weight of all that water finds every weakness in the peat blockage. Suddenly, the plug gives way – and the lake begins to drain like a giant bathtub unplugged. What looked like a placid lake in the morning might by afternoon swirl into a muddy vortex, and by nightfall lie empty except for damp mud and stranded reeds. The speed of the drainage is astonishing. Hundreds of thousands of cubic feet of water can disappear in mere hours. Villagers have reported Loughareema being full at dawn and completely gone by lunchtime. At peak flow, water surges down the sinkhole at a rate of hundreds of litres per second, an unseen torrent roaring through the rock beneath the bog. Once the lake has “run out,” all that remains is a desolate bowl of black peat mud – and the cycle is ready to begin anew.

*Image shows Loughareema when drained

Underground Rivers and Hidden Journeys

Where does the water go after it swirls down the drain? For a long time, that question deepened Loughareema’s mystique. There are no visible caves or swallow holes one can explore safely. Yet scientists have tracked the vanishing lake’s outflow reappearing over a mile away. All that water doesn’t just vanish into oblivion; it travels through a network of submerged passageways in the limestone. Roughly 2.5 kilometres to the northwest, a robust spring gushes out from the base of a hill, feeding the Carey River. This spring is the resurgence – the return to daylight – of Loughareema’s waters. In essence, an underground river connects the lake’s sinkhole to the river valley beyond the hills. When the lake drains, that hidden river swells, flushing water (and the bits of peat and debris that caused the plug) out into the Carey River.

*Approximate location of outlet of Loughareema.

Researchers confirmed this connection using tracing techniques. In one experiment, harmless dye dropped into Loughareema’s sink quickly emerged in the Carey River spring, colouring its waters downstream. The travel time told a story of its own: the water flows through the subterranean channels surprisingly fast. It likely rushes through cracks and conduits in the limestone, perhaps even sizable caverns in parts, propelled by the pressure of the draining lake above. Once the lake basin is drained, a smaller constant trickle continues to flow underground (the sinkhole never stays completely sealed for long). That means even on a dry day when Loughareema lies empty, water is quietly percolating through the chalk beneath, making its way to the distant spring.

What We Know – And What We Don’t

Bit by bit, science has demystified Loughareema’s vanishing act. We know now that the lake’s behaviour stems from a delicate dance between geology and hydrology: the chalk ground riddled with solution holes; the peat and mud that intermittently clog those holes; the episodic floods that overwhelm and flush the system clean. We know that the water doesn’t vanish so much as go incognito – traveling out of sight and resurfacing in a different valley. We even understand why this spot, in particular, became a vanishing lake. It sits at the meeting point of impermeable highlands and soluble lowlands. Rain runs off the hard rocks into this chalk depression, like water funnelling into a sink. It’s a setup found in only a few places on Earth, making Loughareema’s geology genuinely special.

Yet many mysteries persist. One puzzle is timing: there’s no simple schedule to Loughareema’s appearances. Sometimes the lake holds water for weeks; other times it drains within hours of filling. What subtle factors determine how long the peat plug stays in place? Scientists are still investigating whether certain water levels, or perhaps cumulative rainfall over days, trigger the unblock. The exact mechanism of the “flush” is also under study – does the water simply seep around the edges of the blockage until it pops out, or does pressure build like in a siphon, suddenly sucking the plug down? These are not easy questions to answer without peering directly into the sinkhole as it happens.

The layout of the underground conduits is another unknown. There could be a broad cave chamber somewhere under the hills acting as a temporary reservoir, or just a network of hair-thin cracks rapidly carrying water away. Geophysical surveys (like ground-penetrating radar or seismic scans) might someday sketch an outline, but so far the data are limited. Even the origin of Loughareema’s sinkhole is debated. Did it form slowly over millennia as water dissolved the limestone? Or could it have been jolted open by past earthquakes or the weight of glaciers during the Ice Age? With the lake’s drainage system not yet fully mapped, such historical questions remain speculative.

Awe in the Ordinary

Loughareema is proof that some of Earth’s most captivating phenomena don’t need towering peaks or thundering waterfalls. Here is a modest upland lake, far from any tourist hub, that captures imaginations because it defies our everyday experience of how lakes “should” behave. It took modern science years to corroborate what local folklore knew from experience – that the lake truly runs out and returns as if by magic. In peeling back the layers of that magic, we uncovered a tale of rocks and water: of chalk dissolving under bogland, of hidden tunnels and periodic floods, of the intricate and unseen engineering of the natural world.

Here's the video we made on this on the OzGeology YouTube Channel: