Sinkholes

Sinkholes are a natural depression or a hole that is formed when surface layers collapse. Sink holes can vary in size from 3.3 to 2,000 feet in diameter and depth. They can form gradually over time or suddenly. Sink holes could happen naturally or artificially. Natural sinkholes can be formed by water draining or standing still and can also be formed in dry places. The process of forming a sinkhole is from erosion or removal of bedrock, as the rock underneath the ground dissolves or is washed away the ground becomes weak which then cause the ground to collapse. Artificial sinkholes are formed by humans. Artificial sinkholes can form from old mine that was closed off and the ground above it finally gave in, which means it is not a sinkhole because it was not natural. Sinkholes can also occur when water lines break, sewer collapse or old pipes give away.

http://en.wikipedia.org/wiki/Sinkhole

http://en.wikipedia.org/wiki/File:Dead_Sea_sinkhole_by_David_Shankbone.jpg

http://en.wikipedia.org/wiki/File:Sinkhole.jpg