How can dripping water build a stone icicle in a cave?

A stalactite is a mineral deposit hanging from a cave ceiling. It can look like an icicle, but a common limestone-cave stalactite grows by adding calcite, a form of calcium carbonate. It does not need freezing water.

Water moving through soil and limestone can pick up carbon dioxide and dissolve some rock. The water carries dissolved calcium and bicarbonate. When it reaches a cave where conditions allow carbon dioxide to leave the water, the chemical balance changes.

Some dissolved material can then become solid calcite. A little may remain at the ceiling where the drop formed. Repeated deliveries can build a narrow hollow tube and, later, a thicker stalactite as water runs over its surface.

A falling drop may also deliver material to a stalagmite growing upward from the floor. The growth depends on water supply and chemistry, not just on how often drops fall. Water can dissolve rock along one part of its journey and deposit mineral along another.