How and why do sink holes form?
Sinkholes form when water dissolves limestone. The limestone becomes weak, given as water filters down through the ground become acidic and eats away at the aquifer (underground layer of rock which lies below the sand and clay). The erosion of the limestone forms water-filled pockets (which become weak).
These pockets can collapse upon themselves if the pressure from the upper layers exceeds the strength of the top of the pocket.
Other sinkholes happen when the materials (like sand and clay under laid cement, concrete, or asphalts) erode. Again, the supporting materials become weakened, or eroded, and can no longer support the material on top. The materials below will collapse upon themselves and the upper layer will fall into the hole.