In the past year alone, sinkholes in Wales, Surrey and, most recently, Cornwall have revealed a forgotten cave, evacuation of a Love Island contestant’s family and swallowed a Mini, respectively.
What's the world's deepest sinkhole?
Beyond UK shores, the world’s deepest sinkhole is reportedly in Xiaozhai Tiankeng in China. It’s 625 metres long, 537 metres wide, and up to 662 metres deep.
While locals have known about ‘The Heavenly Pit’ for centuries, it only became known to the outside world in 1994 when British explorers attempted to survey its maze-like underground system. Despite the drama, few people have lost their lives in sinkholes, though up to 20 people tragically lost their lives in two Guatemala sinkholes in 2007 and 2010, respectively.
What are sinkholes?
As you’d have seen on the news, the sinkholes are often saucer-shaped hollows and, according to the British Geological Survey, “the result of some kind of collapse or removal of an underlying layer of rocks that used to support the layer of material at the surface”.
How are sink holes formed?
This ‘collapse’ or ‘removal’ is down to myriad reasons. Some sinkholes form when soluble rocks – such as gypsum and chalk, as seen in many a landslide on the Jurassic Coast in Dorset – are slowly dissolved by acidic rainwater or ground water. These are known as ‘solution sinkholes’, and their sides range from a rather shallow slope to sheer vertical. Rivers or streams may also flow into these sinkholes and vanish under the surface. These are called ‘shallow holes’.
There are also ‘collapse sinkholes’ that occur when an underground cavity can no longer support the weight above. These are quite rare though relatively common in parts of South Wales where sandstone rocks overlay cavernous limestone.
And to complete your sinkhole masterclass, you have ‘suffosion sinkholes’. These occur when the soil above a dissolved rock cavity gradually sinks into it, forging a depression. In sandy soil, this happens slowly, while clay soils can build larger cavities that collapse suddenly, known as drop-out sinkholes.
What causes sinkholes?
The causes of sinkholes are often due to heavy, persistent rainfall or flooding, but also building work, leaking pipes – two of the mooted causes of a sinkhole in Bangkok this summer that nearly swallowed a hospital and police station – changes in water table level and mining.
According to research published earlier this year, climate change is cranking up the frequency of sinkholes due to extreme weather events. “Droughts lower the water table, removing subterranean support for the land above,” said lead author Hong Yang of Reading University. “When followed by intense storms or heavy rain, which are also becoming more common due to climate change, the sudden weight and saturation of water can cause the weakened ground to collapse.”
That’s why, theoretically, no residential street is safe from sinkholes. But if you live in an area with soluble rock like limestone, experience heavier rainfall than the rest of the country, know there are tunnels nearby and you notice depressions forming near curbs, we’d recommend you walk around with a hard hat!
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