• Terrestrial helium has two sources: (1) primordial helium that was incorporated in the Earth at the time of its formation and is now derived from sources deep within the Earth, (2) radioactive decay of uranium and thorium which are concentrated in the Earth’s crust. Helium is composed of two isotopes: helium 4, which is produced by radioactive decay, and helium 3, which was created before the Earth formed and was incorporated into the Earth during its formation.