Volatiles -- such as water, carbon dioxide and the noble gases -- come out of the earth's interior through volcanism and may be injected into the mantle from the atmosphere, a pair of processes called mantle degassing and regassing. The exchange controls the habitability of the planet. This transport could not have begun much before 2.5 billion years ago, according to new research that also establishes a range of dates during which the Earth shifted to a net regassing regime.