Some of the earliest maps known were made by the Babylonians, around 2500 B.C.. They were drawn on clay tablets. In Ancient Greece, Hecataeus of Miletus produced what may have been the first book on geography about 500 B.C. By 350 B.C., the Greeks knew that the earth must be round and about 240 B.C., Eratosthenes of Alexandria had almost accurately calculated the distance round the earth.