Weathering creates underground caves and passages in limestone in addition to depressions and other unusual dips and grooves on the surface. Karst is landscape formed from the weathering of limestone.
Limestone is a sedimentary rock created from the remains of dead sea creatures and is predominately made up of calcium carbonate. Many horizontal and vertical cracks run through limestone. These cracks allow water to pass easily through the rock. Water acts as an acid when it contacts calcium carbonate, dissolving the limestone. The dissolved calcium carbonate may drip into underground caves hollowed out from the action of weathering. Sometimes, the calcium carbonate evaporates as it drips from the cave ceiling creating stalactites. If the calcium carbonate drips and evaporates on the ground it creates stumps called stalagmites. Occasionally other unusual features develop from the dripping and evaporation of calcium carbonate such as curtains or columns.
Complex underground passageways develop as limestone weathers. Water flow through these passageways creates immense underground networks of rivers and streams. These water flows are accessed by humans and used as aquifers for water storage and extraction. Many places depend upon limestone aquifers for their water supply. Care must be taken with using limestone aquifers since pollutants as well as rain can easily pass through the limestone.