Francois Mellet, Electrical Engineer: Uranium in nature is a slightly radioactive metal that occurs throughout the Earth’s crust. It occurs in most rocks, in concentrations of 2 to 4 parts per million. Uranium is about 500 times more abundant than gold. It is also naturally present in most soils, as well as in many rivers and in sea water. The earth’s Uranium (chemical symbol U) was apparently formed in supernovae up to about 6.6 billion years ago. Its radioactive decay provides the main source of heat inside the earth, causing convection of molten rock and consequent continental drift.