As a huge sinkhole swallowed a bus in Northwest China this week, we ask what are the causes and whether they can be prevented.
A huge sinkhole swallowed a bus, killing six people in Northwest China this week, sparking an electrical explosion and leaving several missing.
Several people disappeared as the sinkhole spread and 16 people were taken to hospital.
Sinkholes are not uncommon in China and experts cite the country's rapid pace of huge construction work as one of the triggers.
Watch the BBC footage of the latest incident below:
What triggers sinkholes?
Dr Clive Edmonds is a geotechnical specialist and has studied sinkholes for the past 30 years.
Water
"The main trigger for sinkholes is water," says Dr Edmonds.
"In 90% of sinkhole cases, water saturating the ground is the main trigger, known as Karst processes."
"Sinkholes happen when a layer of rock underneath the ground is dissolved by water.
"In regards to China and their sites, I know there there are some Karst problems with limestone deposits."
Geology
The geology of the area has a significant role to play in the formation of sinkholes.
"Sinkholes depend on geology," explains Dr Edmonds.
"Some rocks are more soluble than others - salt deposits for instance dissolve more quickly.
"Gypsum for instance is a very soluble rock. Chalk and limestone deposits may take thousands of years to dissolve but when new cavities occur they can cause spectacular sinkholes."
Humans
"Human development can contribute greatly, especially construction work in urban areas where you may have dense development with roadworks, etc. as in the case of China," says Dr Edmonds.
"The surface becomes impermeable with water collecting over time in drains and sewers - water starts leaking into the ground."
Sinkhole warning signs
Dr Edmonds said that there are sometimes warning signs of a sinkhole.
"First indicators are dips and depression in the ground surfaces, deepening with tiny cracks in pavements or buildings for instance, leading to buildings showing slight movement," he said.
Tunnelling into water-filled cavities or where water bodies can flow catastrophically into an excavation are particularly hazardous.
"In the UK, any such instances have been caused by small cracks in the ground.
"These lead to the onset of building movement and then a breakdown in the water utilities, or leak of a mains water pipe, or a sewer eventually draining water into the ground."
In the UK there has been a long history of mining and water abstraction, which in some areas has left a legacy of man-made cavities.
Can you always predict when a sinkhole is going to appear?
The term sinkhole is, in the UK, often extended to cover the collapse of ground into such man-made features.
However, Tony Bracegirdle, senior partner of the Geotechnical Consulting Group, says that there can be very little surface evidence of a sinkhole, which makes predicting a collapse difficult.
"There are seldom warnings in urban areas," says Tony. "Although sometimes sinkholes develop slowly to the extent that there's sufficient time to restrict access."
"Sinkholes tend to focus in specific geological and topographic conditions and so the hazard can be reasonably quantified in areas where there is a history of recurring sinkhole activity."
"The most common response in high-risk locations is to design works that are insensitive to potential sinkholes," says Bracegirdle.
"[Works that] control surface water, and to take additional precautions to limit water loss from drains and services."
