A good map is like a fine piece of art, the more you look, the more you find. Cartography is the art and science of making maps. The oldest known maps are preserved on Babylonian clay tablets from about 2300 B.C. Cartography was considerably advanced in ancient Greece. The concept of a spherical Earth was well known among Greek philosophers by the time of Aristotle and was advanced by Mercator with his cylindrical projection that is still widely used for navigation charts and global maps. All maps are made according to certain basic assumptions and impart an artistic, religious, historical and/or political bias. During the age of exploration, maps were carefully guarded state secrets, revealed on the penalty of death. Today they provide insight into times past.