Karst hydrogeology has been the subject of much study in the United States. About 15 percent of the conterminous United States has carbonates, gypsum, or other soluble rocks at the land surface. Dissolution of these rocks has formed karst topography at the surface and karst features in aquifers that are a major source of drinking water. Carbonate aquifers, however, are commonly vulnerable to contamination from the land surface. In 1985, about 25 million cubic meters of water per day was withdrawn from carbonate aquifers in the United States. Three fields of current karst research in the United States are water-resources management, sinkhole formation, and carbonate geochemistry and geomorphic consequences. Examples of karst fields where these areas of research are continuing, include Alabama, Florida, and Puerto Rico.