BACKGROUND: 10 things you didn't know about sinkholes

The Sheffield sinkhole.
The Sheffield sinkhole.
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A giant sinkhole which opened up in a Sheffield road has left locals puzzling as to how it happened.

The 8ft deep hole opened up in Hutcliffe Wood Road, Millhouses yesterday afternoon - and theories for the collapse focus on collapsed mine workings.

VIDEO: Huge sinkhole opens up in Sheffield road

But what are sinkholes? And how are they formed?

Here's all you need to know...

1. Sinkholes are caused by a collapse in the surface layer - some are caused by the dissolving of rocks below ground in a chemical process.

2. Holes can vary in size in diameter and depth - from a matter of inches to hundreds of metres deep.

3. They can form gradually over a number of years - or open up with little or no warning.

4. Their formation involves natural processes of erosion or gradual removal of slightly soluble bedrock (such as limestone) by percolating water, the collapse of a cave roof, or a lowering of the water table.

5. Sometimes, a sinkhole may reveal a visible opening into a cave below. The formation can be dramatic because the surface land usually stays intact until there is not enough support before a sudden collapse of the land surface.

6. Sinkholes also form from human activity, such as the collapse of abandoned mines. More commonly, sinkholes occur in urban areas due to water main breaks or sewer collapses when old pipes give way.

7. China is home to the 662-metre (2,172 ft) deep Xiaozhai Tiankeng (Chongqing, China), sinkhole.

8. The deepest water-filled sinkhole in the world—Zacatón— is in Mexico ] with a total depth of 339 metres (1,112 ft)

9. Sinkholes have been used for centuries as disposal sites for various forms of waste. A consequence of this is the pollution of groundwater resources, with serious health implications in such areas.

10. One of the biggest sinkholes in Europe is located at Pouldergaderry in Ireland. The sinkhole is approximately 80 metres (260 ft) in diameter and 30 metres (98 ft) deep with many mature trees growing on the floor of the hole.