The Clarksville Cave has only one known entrance (Lewis). It is a sinkhole that is located a small clearing in the woods of upstate New York (LSRHS).
A sinkhole is commonly, but not always found in areas where limestone is predominant (USGS). As the limestone dissolves from water, which is naturally acidic, it forms a cavity in the ground (USGS). Once this loses support and becomes unstable it becomes a sinkhole (USGS).
Image URL: http://water.usgs.gov/edu/sinkholes.html
clarksvillecaveproject 