When thinking humans first evolved, it was possible to find nearly pure deposits of several metals exposed on the Earth's surface. The most common were gold, silver, and copper. As time passed, the surface deposits were exhausted and men began digging into the ground looking for more. The mining and excavation of the Earth in search of minerals continues today, although the list of things for which we dig has greatly expanded. The discovery of new metals and resources, or new methods of extraction, often turned previously unwanted areas into mother lodes of oil, uranium, platinum, etc.