Limestone is made up of fossils. After small marine animals die, their shells and skeletons build up on the ocean floor. Over time, the layers of fragments press down on each other, squeeze out the water, and recrystallize into solid rock. Sometimes the original shells, which are now fossils, are still visible. These fossils give scientists important clues about ancient life.

Image credits: main image, © AMNH; Rondi Davies: AMNH.