Sinkholes are caused by erosion, or underground caverns when the water table lowers. New sinkholes typically form because of a change in the underground water network. Water supports the ground surface, so when an underground river drains for some reason, the water is no longer there to support the surface. Sinkholes happen when the ground caves in in weak spots. Basically, every sinkhole is the entrance to a cave. There are a few other ways they can form, but those are the kind I have experienced.