My anwser to you is http://veloroutes.org/elevation/
Karst Topography is a chemical reaction about the dissolution of layer(s) of soluble bedrock, clearly, carbonate rock like limestone or dolomite. Acidic water erodes the calcium in the bedrock, which is a main constituent of carbonate rocks in creating caverns. First tiny fractures/tunnels form in the rock and yet, with adequate time and dissolution, this acidic erosion can make huge caverns.
B: a few kilometers below the earth's surface.
