elongated mineral formation which hangs down from a cave ceiling

A stalactite is a form that can be found on the ceiling of a cave. When a drop of water flows down the stalactite, the water evaporates and leaves a mineral deposit of calcite. This slowly increases the length and thickness of the stalactite.

Stalactite that is dripping water

The corresponding formation on the floor underneath a stalactite is known as a stalagmite. Given enough time, these formations can meet. The results are known as columns.