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. When this water that now holds the dissolved rock is exposed to the air in the cave, it releases the carbon dioxide gas, much like when a bottle of soda is opened. As the redeposited minerals build up after countless water drops, a stalactite is formed.