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A large number of collapse sinkholes have been developed in the period [1989][1990][1991][1992][1993][1994][1995][1996][1997][1998][1999][2000][2001][2002][2003][2004] in Famenin and Kabudar Ahang plains, Hamadan province, west of Iran. This paper discusses the Hamadan sinkhole conditions and their mechanism of formation. There is limestone bedrock at the base of a thick cohesive alluvial aquifer in the area. High purity of limestone, considerable porosity and existence of numerous joints and fractures favors high karstification of the limestone. Overexploitation of groundwater during the last decades created a significant drawdown in aquifer water table. Besides, deep wells penetrated to the limestone bedrock and evacuated fine-grained materials of overlain alluvial aquifer. Therefore, large cavities resulted from sand productive wells. Increasing effective stresses have mainly caused the formation of dropout sinkholes or cover-collapse sinkholes. The sinkholes of the plains roughly coincide with the maximum drawdown in the aquifer. Therefore, one of the main factors that has controlled the formation of sinkholes is the water table drawdown in the aquifers. Karstification of limestone bedrock and thick cohesive soil of overlain aquifer enhance the formation mechanism of sinkholes.

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