While it's unusual for a sinkhole to open up beneath a home without warning, as it did Feb. 28 in Seffner, sinkholes in general are not uncommon in Hillsborough County.
CDS Business Mapping of Boston, which maps hazards around the country to help insurance underwriters assess risk, says Hillsborough County ranks third in the list of the top 10 sinkhole-prone counties in Florida.
The company reports that sinkholes have been on the rise in Florida the last few years. Sinkhole property insurance claims increased from 2,360 in 2006 to 6,694 in 2010.
Pasco County tops the list, followed by Hernando County and then Hillsborough.
According to the Southwest Florida Water Management District, sinkholes are an erosion of land. They form when underground water dissolves minerals in the ground, creating a buried hole. Eventually, the hole becomes large enough that the land on top caves in.
Sinkholes are common in land containing limestone because limestone is soft and has water holes like a sponge.