The spelethems that most people are common with is stalactites, stalagmites, columns, flowstone, and popcorn. Stalactites grow from the ceiling of a cave. A stalagmite grows up from the bottom of a cave. It's easy to remember which is which on how to spell and remember, stalactites have a T for top and stalagmites have a G for ground. Speleothems form because of water.
Rainwater seeps through cracks in the ground and through rocks.
It passes through organic minerals it picks up carbon dioxide gas, and it creates carbonic acid. This acid passes through limestone and dolomite. The mineral calcite is dissolved from the limestone and dolomite in which a cave is formed. This water now holds the dissolved rock is exposed to the air in the cave, it releases the carbon dioxide gas, like a bottle of soda being opened. As carbon dioxide is being released, calcite is precipitated on the cave walls, ceilings, and floors.