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What is a sinkhole?

The road initially plummeted in a roughly square shapeLauren Bourke
Nearby homes were evacuated after this sinkhole appeared on the A26 in Kent

A sinkhole has opened up on a main road in Kent, England.

Drivers said they heard a "crunching sound" before the 2m-wide sinkhole appeared on Monday evening.

Work to repair the void - which reaches up to 20m underground - is likely to take weeks.

But what is a sinkhole and where do they come from?

What is a sinkhole?

A sinkhole is a hole in the ground formed when the land collapses away leaving a big hole on the surface.

It happens when the rock underneath is dissolved by water.

Sinkhole in JapanGetty Images
Sinkholes can cause great damage to towns and cities when they happen in built-up areas, such as this sinkhole in Japan in November 2016

They can range in size enormously, as you can see from the videos above and below.

Sinkholes form when rainwater comes into contact with a certain type of soft rock - such as chalk or limestone - and dissolves it, leaving a gap deep in the ground.

The land on the surface then collapses into this gap.

The rainwater is able to dissolve soft rock like this, after seeping through the top layers of soil and having CO2 added to it, which makes it more acidic.

Where do they happen?

Areas on top of land with limestone or chalk foundations are more at risk of sinkholes.

They are pretty common in the American state of Florida because virtually the entire state is built on a limestone platform, but they are rare in the UK.

Sinkhole in FloridaGetty Images
Florida is at a greater risk of having sinkholes because of the foundations of the land on which the state is built

They can also occur in areas where there has been mining activity in the past and so the composition of the land deep underground has been affected.

Even though they are formed over quite a long period of time, it can be rather difficult to predict when they are going to happen.

Check out these videos showing sinkholes all around the world
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A giant sinkhole has opened up on a farm in an area of New Zealand called the Bay of Plenty (May 2018)
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In this video, a fire engine is swallowed by sinkhole (nobody was in the truck and nobody was hurt) (February 2017)
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In February 2017, a sinkhole opened up in the middle of a street in Sydney in Australia
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This stunning aerial video shows the world's deepest underwater sinkhole, according to scientists in China (July 2016)
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In this video, you can see this sinkhole in the middle of a street in California opening up right before your eyes (April 2016)
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This huge sinkhole opened up in the back garden of a house in Cornwall in south-west England (March 2016)