Phosphorescent 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. To image the phosphorescent property the rock sample was illuminated with a very bright UV flash before the camera shutter was opened. The camera was able to capture the light from the sample as the excited electrons fell back to their ground state and emitted light.