Minecraft wiki entry for Pointed_Dripstone, associated text: If a stalactite is hanging from a dripstone block with a water source above, the stalactite grows slowly. In addition, if there is a solid block within eleven blocks below this stalactite, a stalagmite grows on that block toward the stalactite. This growth does not occur if the stalactite's tip is waterlogged, and a stalagmite below does not grow if its tip is waterlogged. They do not grow to more than seven blocks long.