A sinkhole is simply a big hole on the surface of the Earth. It naturally occurs in many parts of the world. Those that are deep underwater are called blue holes. Its mainly caused by the erosion or dissolution of underlying ground. A long process triggered by one or many things underground such as collapsing caves, ground water circulation, and even broken sewer pipes.
One of the latest sinkholes appeared in Schmalkalden, Germany November 2010. It is 40 meters wide, and 20 meters deep. Fortunately, there were no reports of missing or injured people. Only a car was taken by the hole while leaving the rest of the houses that surrounds it intact. The exact reason is unknown, but it was speculated that the area had cavities underneath that resulted from broken rock formations.
One very notable type of sinkholes is the black hole that exists in South Andros and the Grand Bahama Islands. Seen from above as a black river, scientists were surprised that the upper levels are actually very clear. And the rest below were made of black muddy substance. Later it was known that this substance actually contained a huge amount of organisms that feed on sulfur and light!