Abstract Most sinkholes in the Hamedan region central plain in western Iran are developed in partially unconsolidated sediments overlying Oligo-Miocene limestone. Analyses of hydrologic and hydrogeologic conditions from 1988 to 2008 show that conditions were favourable for sinkhole development. These sinkholes developed in groundwater discharge areas near the base of karstic Oligo-Miocene limestone in zones with major groundwater circulation, thick overburden and a close proximity to deep-source carbon dioxide gas. High carbon dioxide partial pressures, caused by rising gases from pneumatolytic and hydrothermal phase sources, renew the aggressiveness of the water in the vicinity of the discharge zone. The acidic water dissolves limestone, especially along the fractures and joints. Moreover, the presence of large conduits below the surface allows for the transport of unconsolidated sediments and the continual development of sinkholes.