https://www.vice.com/en_us/article/bvjn5v/earths-core-is-leaking-and-scientists-finally-have-a-theory-why

Scientists think they may have solved the mystery of how helium-3, a rare element that has attracted interest as a potential fuel source for nuclear fusion reactors, is leaking out of Earth’s core, reports a new study. 

The research proposes that this ancient substance seeps from the core into the planet’s outer layers, the mantle and crust, as a result of reactions with magnesium oxide. The discovery could help probe Earth’s mysterious innermost regions, and has implications for understanding how life emerged on Earth and whether it might exist elsewhere in the universe.