sinkholes

A sinkhole is a depression in the ground that is caused by the collapse of surface layers. Most sinkholes are the result of karst processes, or the dissolution of soluble rocks. These sinkholes happen gradually, while collapse sinkholes can form within hours. Sinkholes can also result from human activities such as mining. In Colorado Springs, abandoned coal mines from the late 1800s have collapsed in places, resulting in sinkholes throughout the area.