Helium-3

Helium-3 (3He) is a light, stable isotope of helium. It has two protons and one neutron. Helium-3 was discovered in 1939.
Helium-3 occurs as a primordial nuclide. It escapes from the Earth's crust into the atmosphere. It then escape into outer space over millions of years. Helium-3 is also thought to be a natural nucleogenic and cosmogenic nuclide. The amount of helium-3 is thought to be greater on the Moon than on Earth.