It's hard to imagine anything more terrifying and unexpected than previously solid ground opening up beneath your feet.
It's what happened in Walsall on Thursday, May 21, when part of a road in the Midlands town collapsed.
This frightening phenomenon takes place all over the world, and for a variety of reasons.
But what are sinkholes? And what causes them?
What are sinkholes?
A sinkhole is a hole in the ground caused by a collapse in the levels of earth and rock below the surface.
They can be very small - around a metre in diameter and depth - or very large. One of the biggest sinkholes ever recorded was around 650 metres deep and nearly 540 metres wide.
Sinkholes can take place on a city street or in remote countryside.
What causes sinkholes?
Natural sinkholes take place when rock deep below the surface is eroded slowly by rainwater seeping down from the surface.
Over time the acidic rainwater wears away the rock, creating voids beneath the surface.
Soil from above falls down to fill the gaps, creating more holes. Eventually - and this could take many years - the surface level collapses, causing a sinkhole.
Sand collapses gradually, creating a sinkhole over time, according to the (BGS). But clay can hold out for years, meaning that when the collapse comes it can be dramatic.
Heavy rainfall can speed up the creation of a sinkhole significantly.
Other causes of sinkholes are when a deep underground cave collapses, undermining the levels above it. This is common in Wales and Ripon in Yorkshire.
Where are they found?
Sinkholes can take place in many parts of the world. There are several regions that are particularly prone to sinkholes, like in Florida and parts of southern Italy.
There are areas throughout the UK that see sinkholes relatively regularly, according to the British Geological Survey (BGS), including parts of Hampshire, Wales and the Peak District.
The most susceptible part of the country is Ripon, which has seen several sinkholes that have damaged people's housing and property.
But most sinkholes in the UK are relatively small.
Can they be affected by human activity?
There are several human activities that can help trigger sinkholes, the BGS has found.
Leaking pipes can be a major factor, while a swimming pool emptying has also been reported to have helped caused a sinkhole.
Building works that don't have enough support can be a trigger, while areas that have had significant mining may also be at risk.
What to do when a sinkhole develops
Sinkholes can be dangerous, although deaths appear to be relatively rare.
If a sinkhole has appeared, you should cordon off the area if it is safe to do so, to keep people away, the BGS has said.
Let relevant authorities like the emergency services know immediately.
Try to turn off anything that could be worsening the collapse, like pipes that may be leaking.
