The sedimentary rock limestone is composed of the mineral calcite, which is water-soluble, meaning it will dissolve in water that is weakly acidic. In humid areas where limestone is found, water dissolves away the rock, forming large cavities and depressions which vary in size and shape. As more dissolution occurs, the caves become unstable and collapse, creating sinkholes. These broad, crater-like depressions are typical of karst topography, named after the Karst region in Slovenia. Karst topography is characterized by sinkholes, sink lakes (sinkholes filled with water), caves, and disappearing streams (surface streams that disappear into a sinkhole). Living in karst topography poses its challenges, and approximately one-fourth of Americans in the lower 48 states live in these regions. Sinkholes can appear rather rapidly and cause great damage to any structures above them.