Are sinkholes becoming more common in the UK?
Train services disrupted after 13ft-deep pit opens above a sewer in south London
Commuters have faced widespread disruption after a 13ft-deep hole opened above an active sewer in Forest Hill in south London.
Engineers are still trying to fix the problem, which appeared yesterday. Network Rail said it was moving more than 50 tonnes of ballast to shore up the track.
Danny Leamon, the senior operations manager at Thames Water, said they had specialist teams working through the night. "This is an extremely complex job as the pipe is in a difficult location in the tracks and six metres below the ground," he said.
Sinkholes have been reported across the country over the past couple of years, from Yorkshire to Cornwall, causing problems for local communities. They have swallowed cars and even ravaged people's homes.
But why do they happen and are we making them worse?
Why do sinkholes happen?
Sinkholes are a naturally occurring phenomenon when water gradually dissolves soluble bedrock, forming a cavity under the ground over thousands of years. Loose sedimentary rock from nearer the surface level gradually falls into the hole, until the surface later is unable to sustain itself any longer, and collapses.
Dr Anthony Cooper of the British Geological Society compares the process to the motion of an egg timer. Periods of persistent heavy rain or flooding are most likely to expose these holes, but, conversely, sudden drainage of groundwater can also cause them.
"A water-filled cavity beneath a bridge of material will actually support that bridge to a certain degree," Dr Cooper told The Guardian. "If you the pump a lot of water out, all of a sudden that material, which is water-saturated and heavier, suddenly collapses in."
Another type of sinkhole, called a crown hole, is caused by human activity, with abandoned mines and underground sewage works at particular risk of collapse. In the case of the St Albans sinkhole, local archaeologist Roger Miles told the BBC that the town's disused clay pits had caused fissures in the ground.
Sinkholes in the UK are rarely more than a few dozen feet deep, dwarfed by the likes of the Red Lake in Croatia – which, at 1,740ft, is Europe's deepest sinkhole. The largest sinkhole in the world is the 2,172ft deep Xiaozhai Tiankeng in China.
Some parts of the Britain are more vulnerable than others, as the likelihood of a sinkhole forming depends on how soluble the underlying rock is. "The South East is fairly susceptible to sinkholes [because of chalk]," Dr Cooper told the Daily Telegraph. "From the south up to Norwich, that whole area is susceptible to both natural sinkholes and those caused by mining."
Are we making things worse?
Though sinkholes are closely linked to rainfall, humans can exacerbate the process. Water extraction, for whatever reason, is often to blame, says the The Guardian's Karl Mathiesen. "Anything that has the potential to divert water into weak points beneath the earth will accelerate the creation of the pits into which houses, cars and unfortunately people sometimes fall," he says.
But Mathiesen also points out that the timescale for many of these events is thousands of years, meaning that most sinkholes began forming long before humans started interfering. "These are primarily natural phenomena, which humans can occasionally influence," he concludes.
Are sinkholes becoming more common?
With widely-circulated reports of sinkholes in St Albans, Hemel Hempstead and High Wycombe in the past few years, as well as stories of London's exploding pavements, it can seem that there's some kind of geological uprising taking place in the UK.
It's true that more major sinkholes are appearing around the UK. In early 2014, scientists recorded a five-fold increase in the number of sinkholes occurring over a particularly rainy winter – and the evidence suggests there are more to come.
Ongoing climate change raises the likelihood of extreme weather, meaning the torrential rain and flooding conditions which often lead to the exposure of sinkholes are likely to become increasingly common.
But the good news is that the average person is still extremely unlikely to be swallowed by one. Despite an estimated 32,000 natural sinkholes scattered around the country, they have not been responsible for a single death.
Is it possible to predict where and when a sinkhole will occur?
Wet weather leading to saturated ground is the primary catalyst for the appearance of natural sinkholes in the UK.
Because they are usually caused by the erosion of bedrock, sinkholes form more easily in areas that stand on chalk, gypsum or salt rock, which dissolve quicker than other types of ground. This makes the south east particularly susceptible, as well as the Isle of Wight and parts of Yorkshire.
Surveying can establish whether a property is built over any manmade features that increase the risk of a sinkhole, like a disused mine or tunnel. When it comes to naturally occurring sinkholes, it becomes far harder to predict.
While the technology to examine bedrock in detail does exist, it would be impractical to uncover and monitor all the fissures and cavities that might eventually form a sinkhole.