Southern China accommodates one of the largest karst units in the world. The limestone that forms this unit has been dissolved by water over countless centuries. In certain regions, the limestone has deposits of aluminum sulphate, which are released as the limestone dissolves and gives the drainage systems their characteristic blue color. This is a natural bridge, which used to be part of a cave hundreds of years ago. Dissolution from rainwater has broken down the system and this arc is all that remains.