Recently, there was a sinkhole in Toledo, Ohio.
Woman Rescued After Driving Into Sinkhole (Video - Ohio)
http://news.yahoo.com/video/raw-woman-rescued-driving-sinkhole-004643
Could that sinkhole in Toledo possibly do us a favor and take out the whole city?
The link is dead, but if I found the correct story, the sinkhole there was due not to Karst limestone geology but to erosion caused by a sewer pipe gone bad.