Speleothems , sometimes referred to as formations or decorations, are cave features formed by the deposition of minerals. The speleothems with which most people are familiar are stalactites and stalagmites.
Stalactites grow down from the cave ceiling, while stalagmites grow up from the cave floor.
Speleothems actually form because of water. Rainwater seeps through cracks in the rock. As it passes through organic material, it picks up carbon dioxide gas, creating carbonic acid. This weak acid passes through joints and cracks in limestone. The mineral calcite is dissolved from the limestone rock in which a cave is formed.