With a natural abundance exceeding 99.999%, 4He is pretty much the only game in helium town. Yet variations in 3He levels released from rock formations reveal plenty about an area’s geological history. Scientists use the 3He/4He isotope ratio to date groundwater and to construct models that describe geological activity. Jacob B. Lowenstern of the U.S. Geological Survey and coworkers measured helium gas emission rates and isotope concentrations at Yellowstone National Park, a geological hot spot famous for its geysers. They report that emission rates of 4He exceed by orders of magnitude the rate at which conventional wisdom indicates that isotope is created in Earth’s crust via α-decay of uranium and thorium (Nature 2014, DOI:10.1038/nature12992). To explain the discrepancy, the team proposes that helium accumulated for hundreds of millions of years in rocks beneath Yellowstone and has been liberated only recently—over the past 2 million years—by intense geological activity.