Helium gas is renowned for its ability to diffuse through materials quickly. Why is it, then, that we find an abundance of helium in certain rock crystals—that it has not managed to escape from them? This has significant implications for the dating of rocks using radioactive decay. That’s because helium is formed when some radioisotopes decay, and therefore lots of helium suggests lots of decay has occurred. Moreover, if lots of decay has occurred, it also suggests that the rock is very old. However, Nuclear Physicist Dr Russell Humphreys realised that lots of helium trapped in the rock crystals that had not had time to diffuse out means that the rocks were actually young. Dr Humphreys concluded that nuclear decay rates must have been dramatically faster in the recent past, and that is why rocks that are actually only thousands of years old are commonly wrongly dated using radioisotope decay as millions or billions of years old.