A sinkhole is a circular depression, typically funnel-shaped, that can form in a karst area. Karst is a type of topography formed on carbonate rock such as limestone or dolomite, and characterized by sinkholes, caves and open-channel groundwater flow. In Maryland, karst areas occur in Baltimore, Carroll, Washington, and Frederick Counties, with less extensive areas in Allegany County. Depressions that form on karst areas may be sinkholes, however, every depression or hole in the ground isn’t necessarily a sinkhole. Depressions in the land may also be a result of rotted tree stumps, collapsed underground structures such as old spetic tanks, stormwater runoff, and leaking underground pipes. True sinkholes do not form in areas underlain by hard, crystalline rock present in central and western Maryland nor in the unconsolidated sediments of Maryland's Coastal Plain (area approximately east of I-95).