Phosphorescence of limestone in stalagmite layers. Stalagmites are mineral deposits made from limestone that form over thousands of years from minerals carried in the water flowing beneath the Earth's surface. As this water slowly drips through the walls and roofs of a cave system, the minerals precipitate out and form the layers shown here. Stalactites hang down from cave roofs, while stalagmites grow upwards from the ground. Limestone will phosphoresce (glow in the dark) after exposure to intense light. Image #1 of 2 in sequence.