Sinkholes: slightly acidic groundwater dissolves limestone or similar rock that lies beneath the soil creating a large void or cavities. When the overlying ceiling can no longer support the weight of the soil and whatever is on top of it, the earth collapses into the cavity.” (NBS News.)
Here are some photos of the more memorable ones.
And the mother of all sinkholes:
This sinkhole is 60 feet by 300 feet, and swallowed up a three-storey apartment building. Pumice fill, rather than limestone, was said to be the culprit.
Talk about feeling the earth move.