Tiny coccolithophores have had a big impact on the planet over time. Though they are single-celled, these photosynthesising organisms are enclosed in a mosaic, or cage, of microscopic plates that make many very beautiful to look at. The plates are made of calcium carbonate, which the coccoliths pull from the surrounding water. As these small organisms live and die in their trillions, they bequeath their tiny plates to the ocean floor where they form rocks such as chalk. Over geological time, coccoliths have removed significant amounts of the greenhouse gas carbon dioxide from the atmosphere, helping to keep Earth cool as the sun grew hotter.