Both stalactites and stalagmites are conical-shaped mineral deposits formed in caverns by the dripping of mineral-rich water. The difference is simple: stalactites hang from above, while stalagmites grow from the floor up. Both words can be either count nouns (e.g., there were stalactites in the cave) or mass nouns (e.g., there was stalactite in the cave).
And watching a 3D movie underground is a good way to crack your forehead on a stalactite. [Cinematical]
All that has kept parts of the streets covered in a muddy mulch, with the occasional ice stalagmite poking through. [New York Times]
They hang like frozen stalactites from gutters and eaves, slowly enclosing houses behind curtains of ice. [Auburn Citizen]
Inside, there is a small altar, a flame still burning; other spent candles sit on top of stalagmites deeper in. [Telegraph]