Stalactites and stalagmites are mineral formations that occur in caves. They are both types of speleothems, which are cave features formed by the deposition of minerals. The main difference between stalactites and stalagmites is their position in the cave. Stalactites hang from the ceiling of a cave and are icicle-shaped formations that are produced by the precipitation of minerals from water dripping through the cave ceiling. Most stalactites have pointed tips. On the other hand, stalagmites grow up from the floor of a cave and are upward-growing mounds of mineral deposits that have precipitated from water dripping onto the floor of a cave. Most stalagmites have rounded or flattened tips.

Both stalactites and stalagmites are formed by the same process. Rainwater seeps through cracks in the rock and picks up carbon dioxide gas, creating carbonic acid. This weak acid passes through joints and cracks in limestone, dissolving calcite 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 carbon dioxide is released, calcite is precipitated (redeposited) on cave walls, ceilings, and floors. As the redeposited minerals build up after countless water drops, a stalactite is formed. If the water that drops to the floor of the cave still has some dissolved calcite in it, it can deposit more dissolved calcite there, forming a stalagmite.