But there have always been those who claim that oil is a natural substance that forms automatically in the Earth's mantle. They say that it is virtually everywhere, if you can drill deep enough to tap it. Proponents of so-called "abiotic oil" claim that the proof is found in the fact that many capped wells, which were formerly dry of oil, are found to be plentiful again after many years, They claim that the replenished oil is manufactured by natural forces in the Earth's mantle.