Why do holes keep appearing?

http://www.bbc.co.uk/news/uk-england-32433292

Version 0 of 1.

Two people have "disappeared" into a hole that suddenly appeared in a south-west London pavement earlier. But why do holes suddenly appear in suburban streets? And is it happening more frequently now?

Passers by were shocked when they heard a loud scream and saw a woman and her shopping trolley vanish into the void outside a Fulham cafe earlier on Thursday.

However, the accident is just the latest example of a hole suddenly appearing in the ground.

Sometimes it happens in remote areas, such as when an enormous 160ft (49m) wide hole appeared in the Peak District in December 2013.

It also happens in built-up areas, such as when a hole caused a busy road junction to close in Southport, Merseyside, last month.

After examining photos of the hole in Fulham, two experts said it appeared the man and woman had fallen into a basement that was under the footpath.

Structural collapses like this are often caused by water erosion - such as from rainfall or a leaking pipe.

"Potentially if water gets underneath the footpath it can weaken the material a little bit," said Dr Rob Evans, a lecturer in civil and geotechnical engineering at Nottingham Trent University.

Dr Evans said it was not possible to determine the cause for definite just by looking at photographs.

However, Peter Styles, professor of applied and environmental geophysics at Keele University, agreed that water was a likely factor.

"In a lot of these cases what happens is that heavy rainfall washes the soil away," he said.

"This is true of both natural and manmade holes in the ground."

Manmade holes or "cavities" in the ground are often caused by mining.

They can appear in remote or residential areas, such as this large hole which appeared in a Hertfordshire street in February 2014.

It can even happen in London, which has been mined for chalk in some areas.

Dr Evans believes we are more likely to become aware of holes appearing now because most people carry mobile phones with cameras.

"I would imagine when it does happen, within a few seconds you get photos on Twitter or Instagram, and it's quite a shocking thing to see a big hole in the footpath," he said.

"In the old days, a photographer would maybe go along a day later and get a photo of them being repaired."

But Prof Styles believes holes are appearing in the ground more frequently nowadays.

It happens "every few months to a year" in the UK, he says, but "very frequently globally", at least weekly, if not daily.

"It's partly to do with climate change because we are tending to get these very heavy downpours rather than steady rain," he said.

"It doesn't drain away particularly well and it can wash away soil and support from these cavities."