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Helium-3 is part of the solar wind that has buffeted the solar system for billions of years. Since the moon has no magnetic field or atmosphere to speak of, helium-3 atoms aplenty have become embedded in its surface.

 

What makes helium-3 potentially important is that it's an ideal fuel for nuclear fusion, which some believe could generate enormous amounts of clean, cheap energy. Just 40 metric tons of helium-3—about what would have fit in the cargo bays of two of our now-defunct space shuttles—could supply the world's electrical needs for three months, giving helium-3 a value of about $14 billion per ton. A million-plus tons of helium-3 are thought to litter the lunar surface—about $13.6 quadrillion worth.