How does a sinkhole form?

Sinkholes are formed when the land surface above collapses or sinks into the cavities or when surface material is carried downward into the voids.

Sinkholes mainly occur in what is known as ‘karst terrain’; areas of land where soluble bedrock (such as limestone or gypsum) can be dissolved by water. With cover-subsidence sinkholes, the bedrock becomes exposed and is gradually worn down over time, with the holes often becoming ponds as the water fills them in.