books.google.com - What is a sinkhole? Far from being just a hole in the ground, a sinkhole is a very special type of natural phenomenon. Born of the interaction between water and rock, sinkholes appear anywhere rock dissolves easily. Some sinkholes happen gradually over hundreds of years. Other sinkholes open up almost...http://books.google.com/books/about/Sinkholes.html?id=Z5SWpk-38eYC&utm_source=gb-gplus-shareSinkholes
