The UK seems to be suffering from a plague of sinkholes. In the past month alone a 15ft hole has opened up on the M2, a 30ft crater swallowed a car in High Wycombe and in Hemel Hempstead, homes were evacuated after a sinkhole 35ft across opened up in a residential street.
Meanwhile in the US, an even bigger sinkhole (some 40ft wide and 20ft deep) swallowed up eight rare cars in a Corvette Museum. Thankfully, none of these incidents caused any injuries - but the thought of the ground opening up without warning is still justifiably terrifying.
But what is a sinkhole and why does it seem that there are suddenly so many of them about?
What is a sinkhole?
A sinkhole is essentially any hole in the ground created by erosion and the drainage of water. They can be just a few feet across or large enough to swallow whole buildings. Although they’re often the result of natural processes they can also be triggered by human activity.
What are the different types?
There are two basic types, those that are created slowly over time (a cover-subsidence sinkhole) and those that appear suddenly (a cover-collapse sinkhole). Naturally, it’s the latter type that create headlines, but both varieties are formed by the same basic mechanism.
Sinkholes around the world
Sinkholes around the world
1/9 Dunedin, Florida
The rear portion of a residential home is consumed by a sink-hole November 14, 2013. According to reports, the large sink-hole began to form between two houses the morning of November 14, and grew to size of about 30 feet wide by 30 feet deep
2/9 Clermont, Florida
A building sits partially collapsed over a sink-hole at Summer Bay Resort near Disney World on August 12, 2013. The 40 to 60 foot sink-hole opened up under the resort building reportedly begining late August 11 into early August 12. There were no injuries or deaths reported
Gerardo Mora/Getty Images
3/9 Shenzhen, China
Rescuers prepare to move a dead body found in a sink-hole on a road on May 21, 2013. Five people died when a 33 feet wide sink-hole opened up at the gates of an industrial estate in Shenzhen, the southern Chinese boom town neighbouring Hong Kong
4/9 Chicago, Illinois
Workers prepare to pull a truck from a sink-hole that opened up on a residential street in the South Deering neighborhood on April 18, 2013. The driver of the truck was hospitalized after driving into the 15-feet-deep hole while on his way to work. Two other vehicles were also swallowed by the sink-hole
Scott Olson/Getty Images
5/9 Guangzhou, China
Workers use machinery to fill in a sink-hole that buildings collapsed into near a subway construction site. The hole measured about 1,000 square feet across and was around 30 feet deep, but no one was killed, according to a state media report
6/9 The Dead Sea, Israel
sink-holes created by the drying of the Dead Sea, near Kibbutz Ein Gedi, on November 10, 2011
Menahem Kahana/AFP/Getty Images
7/9 Guatemala City, Guatemala
A man inspects a sink-hole formed in a house on July 19, 2011 in the north of Guatemala City. When neighbors heard the loud boom overnight they thought a cooking gas canister had detonated. Instead they found a deep sink-hole the size of a large pot. The sink-hole was 40 feet deep and 32 inches in diamete
Johan Ordonez/AFP/Getty Images
8/9 Beijing, China
Workers use excavators to fill in a sink-hole which occured overnight on Shiliuzhuang road, in Beijing on April 26, 2011. A section of the road collapsed beneath a truck, slightly injuring the driver and a passenger, who both jumped out of the vehicle before it sank into the hole
9/9 Chevy Chase, Maryland
Utlity workers examine the scene of a car caught in a sink-hole caused by a broken water main, which collapsed part of Friendship Blvd. on December 3, 2010. No one was reported injured in the accident
Logan Mock-Bunting/Getty Images
What causes them?
Sinkholes mainly occur in what is known as ‘karst terrain’; areas of land where soluble bedrock (such as limestone or gypsum) can be dissolved by water. With cover-subsidence sinkholes the bedrock becomes exposed and is gradually worn down over time, with the holes often becoming ponds as the water fills them in.
With a cover-collapse sinkhole this same process occurs out of sight. Naturally occurring cracks and small voids underneath the surface are hollowed out by water erosion, with a cover of soil or sediment remaining over the top. Eventually, as the hole expands this cover can no longer support its own weight and suddenly collapses to reveal the cavern underneath.
Why are there so many sinkholes now?
Most of the sinkholes we are seeing at the moment are at least indirectly created by human activity. They're occuring just to the sides of human constructions where rain water has been concentrated on a particular patch of ground in the form of run-off from roofs and tarmac.
However, these local factors wouldn't matter if it wasn't for the wider picture. The South East (where most of the sinkholes have appeared) has not only suffered one of the wettest winters in recent decades, but is also natural sinkhole country - most of the bedrock is the soluble chalk.
How bad have we got it?
Despite the dramatic nature of cover-collapse sinkholes they're not as dangerous as they look. Their effects are localized and once they've appeared they can be safely dealt with. However, this isn't to say that sinkholes are safe by any means.
In 2010 one of the most devastating sinkholes in recent times hit Guatemala City. An area approximately 65ft wide and 100ft deep collapsed, swallowing a three-storey factory and killing 15 people. The sinkhole was caused by a number of factors including an influx of water from Tropical Storm Agatha and leakage from a local sewerage pipe.
In Florida last year a sinkhole also proved deadly after it opened up within a detached bungalow and swallowed the sleeping body of 37-year-old Jeffrey Bush. Bush's body was never recovered from the 30ft wide, 20ft deep hole.
What about the rest of the world?
But even the example above is small compared with sinkholes from the rest of the world. The deepest we know about is the Xiaozhai tiankeng in China. Tiankeng is the local term for large sinkholes and translates literally as 'heavenly pit'. This particular example in the Chongqing district is a staggering 662m deep and 626m wide.
Other notable sinkholes include Sima Humboldt in Bolivia, a crater 314m deep and formed from extremely resistant sandstone; the Great Blue Hole (above), a perfectly round hole in the middle of an atoll which is 124m deep; and Crveno Jezero in Croatia, a 530m deep sinkhole with nearly vertical walls.