The death of a Florida man is bringing the subject of sinkholes to the forefront once again.  Many are expressing disbelief at the fact that they are considered a relatively common occurrence, especially in the Sunshine State.

But what causes sinkholes, and why are they so common?

Sinkholes are considered a naturally-occurring part of the landscape, especially in Florida where the ground is comprised of "karst," or an irregular limestone geological base which has eroded over time, leaving fissures in its foundation.  Over time the fissures give way and - usually gradually - permit sand and soil to seep through naturally, part of nature's plumbing system.

However, as stated on the Pennsylvania Department of Environmental Protection's website, "...human influence causes sinkholes to occur where they might not naturally have happened. Or, they may occur sooner or more abruptly than under natural conditions..." Excessive weight on the surface, e.g. that imposed by a home or other structure, can speed up the process of erosion.

More common, especially in states like Florida or Pennsylvania, are sinkholes which give way gradually, which are able to be periodically refilled by man.  In fact, hundreds to thousands of sinkholes appear in Florida in any given year.

The United States Geological Survey states that "...their increasing frequency corresponds to the accelerated development of ground-water and land resources."

In Florida there are special designations for sinkhole probability zones within the state.  These are used by insurance companies and it is usually mandated that this insurance is available for home and property owners.

Featured in the picture at the top of this story is a 22-ton Los Angeles Fire Department fire truck that had gotten stuck in a sinkhole on September 8, 2009 in the Valley Village neighborhood of Los Angeles, California.  The photo was taken by David McNew of Getty Images.

The picture below is from 12 June 2002.  A sinkhole measuring 150 feet wide and about 60 feet deep is shown a day after it opened in Orlando, Florida.

Chris Livingston, Getty Images

In the photo below a car sits in a sinkhole in Chevy Chase, Maryland on December 3, 2010:

Logan Mock-Bunting, Getty Images