Speleothems are depositional features in the cave. They are also known as formations or cave deposits. They do not occur in caves until a cave has an opening to the outside world. Openings allow for the deposition of calcite.
Two processes create speleothems: evaporation and loss of carbon dioxide. The first process, evaporation, occurs when dry outside air enters the cave, evaporating the water and leaving behind the minerals. This process is not common except near entrances and it creates the formation known as cave popcorn. The second process occurs when water entering the cave loses carbon dioxide. When water from the soil enters rock, it is under more pressure. When it reaches the inside of the cave, the pressure reduces and the carbon dioxide can escape. This reduces the acidity of the water, causing water to be supersaturated in calcite, which settles out of the water. This is the slower of the two processes, but is dominant throughout most of the cave.