Helium-3 (He³)
The moon: The next frontier for excavation.
"When the solar wind, the rapid stream of charged particles emitted by the sun, strikes the moon, helium 3 is deposited in the
powdery soil. Over billions of years that adds up. Meteorite bombardment disperses the particles throughout the top several
meters of the lunar surface." Julie Wakefield
Dustin Hutcherson, Phys. 102 Energy
Background picture from AVARUUS website