What causes the formation of most caves as well as stalactites and stalagmites?

Running water, and the acids from dissolved gases, slowly carve out the stone to form an erosive cave. Where limestone is present above the cave, it will drip out as calcium bicarbonate in water, which reverts to particles of calcium carbonate in air. If the water carries to the floor of the cave, stalagmites (limestone pinnacles) are formed. If the calcium precipitates before it can drip, an "icicle" of limestone, a stalactite, hangs from the ceiling. Usually these forms are created simultaneously, and may merge to form columns or sheets of columns.

Unlike their icy counterparts, stalactites and stalagmites take hundreds to thousands of years to form, at an average of 5/1000ths of an inch (.13 mm) a year. A stalagmite in an Oregon cave has grown less than 1/4 of an inch since it was broken off 100 years ago!