Sinkholes

An Illinois man has survived an 18ft (5.4m) fall inside a sinkhole while he was golfing, two weeks after another sinkhole swallowed a person in Florida. Mark Mihal, 43, was investigating an unusual depression when the earth gave way on the 14th hole of the fairway in Waterloo, Illinois.   Friends managed to bring the mortgage broker to safety with a rope, and he escaped with only a sore shoulder. 

- News report 

Mark Mihal's golf partners rescue him from the 18ft-deep sinkhole with a rope 

With the report of another sinkhole that swallowed a man, it felt a bit like a sci fi movie where we are being attacked by underground aliens. So what are sinkholes and how do they get formed? 

Sinkholes, or dolines, often take thousands of years to form and vary hugely in size. The deepest is China's Xiaozhai Tienkeng at 662m. The Qattara Depression in Egypt is roughly 80km by 121km in surface size. Sinkholes can also be only a few metres in diameter. 

They are usually the result of what are known as Karst processes. They happen when a layer of rock underneath the ground is dissolved by acidic water. Soluble rock - often limestone or gypsum - beneath top soil layers erodes over many years by acidic water, creating a cavity below the surface. The top level of soil collapses into the eroded cavity when it can no longer support the weight above. Florida is particularly prone to sinkholes as the entire state has limestone underneath it. 

Typically rainfall seeps through the soil, absorbing carbon dioxide and reacting with decaying vegetation. As a result, the water that reaches the soluble rock is acidic. The acidic water causes the erosion of the soluble rock layers beneath the surface - eventually creating cavernous spaces. The soil or sand over the limestone collapses into a sinkhole when it is no longer supported because of the cavity below. This final collapse of the surface might take anything from a few minutes to several hours. 

Some amazing sinkholes:

"Nature is the mother and the habitat of man, even if sometimes a stepmother and an unfriendly home."

John Dewey (1859-1952)