Stalactite, and stalagmite, rocky structures found in limestone caves. Rainwater (containing atmospheric carbon dioxide) percolates through the rocks above limestone caves, dissolving some calcium carbonate, a major component of limestone. Upon reaching the cave, some of the water that drips from the roof evaporates, leaving a little of the calcium carbonate as calcite on the roof; repetition of this process forms a stalactite. The water that continues to drip to the floor leaves a small amount of calcium carbonate (as calcite) on the floor; repetition of this process forms a stalagmite. The two structures often meet to form pillars.