Geology.about.com | Fine clean sinkhole in the desert of Oman is a good example of this landform

Sinkholes are closed depressions that arise in two events: groundwater dissolves limestone, then the overburden falls into the gap. They are typical of karst.

This fine clean sinkhole in the desert of Oman is a good example of this landform. There are many, many more sinkholes in places like the eastern United States where limestone and water are widespread, but they are never this easy to see.

Sinkhole — courtesy Trubble of Flickr