The landscape that develops in a region underlain by limestone or other soluble rock has certain pronounced characteristics and is given a special name - karst.
See US Map of Karst areas
Such a limestone terrain in southeast Minnesota is pocked with large and small closed depressions called sinkholes. Most sinkholes form when soil is transported through solution channels in limestone. However, some form when the roof of a shallow underground cavern collapses.
Sinkhole formation is still an active process. Florida and Texas especially are areas where sinkholes are a common geologic hazard. In Minnesota this hazard is in the southeast corner of the state.
When sinkholes occur they cause damage to houses, foundations, roads and farmers fields.
Some sinkholes are large and can severely disrupt the fields productivity. They are also direct conduits of water to groundwater. In the past many sinkholes were used for dumping trash, vehicles and other garbage. This contributed directly to pollution of groundwater and is no longer allowed. Today when sinkholes are found they are filled with soil or cement to protect the groundwater if at all possible.
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