ST Explains What is a sinkhole and how is it formed?

A 48-year-old woman fell into an 8m-deep sinkhole in Malaysia on the morning of Aug 23.

- Christie Chiu

The Indian national was visiting Malaysia and walking in the Jalan Masjid India area in Kuala Lumpur when a section of the sidewalk suddenly caved in.

Only a pair of slippers believed to belong to the woman has been recovered so far, with search and rescue efforts now hindered by strong water currents running under where the sinkhole appeared.

The Kuala Lumpur police chief, Datuk Rusdi Mohd Isa, said the team will need to proceed cautiously and methodically as any disruption could lead to flooding, and that gas pipes underground are also complicating the rescue efforts.

But what are sinkholes, and how do they form so suddenly? The Sunday Times explains.

Q What is a sinkhole?
A While the word sinkhole is often used to describe any large hole that opens up in the ground, using it as a catch-all term is technically incorrect.

Strictly speaking, The Geological Society of London says, sinkholes are "a natural surface depression caused by dissolution of soluble rocks at depth".

According to the British Geological Survey (BGS), this means these typically saucer-shaped hollows are caused by the collapse or removal of an underlying layer of rocks needed to support the surface layer of material.

But the shape, size and depth of a sinkhole can vary greatly depending on the composition of the rocks and different interactions which caused it.

Q How does a sinkhole form?
A Sinkholes are commonly formed when the supporting underlayer of rock is naturally dissolved by acidic rainfall or groundwater circulating through it, causing spaces and caverns to develop underground, according to the US Geological Survey.