Radiogenic He-4 is produced by the decay of uranium and thorium in the Earths mantle and crust. From here, it is degassed to the atmosphere(1-5) and eventually escapes to space(1,5,6). Assuming that all of the He-4 produced is degassed, about 70% of the total He-4 degassed from Earth comes from the continental crust(2,-5,7). However, the outgoing flux of crustal He-4 has not been directly measured at the Earths surface(2) and the migration pathways are poorly understood(2-4,7,8). Here we present measurements of helium isotopes and the long-lived cosmogenic radio-isotope Kr-81 in the deep, continental-scale Guarani aquifer in Brazil and show that crustal He-4 reaches the atmosphere primarily by the surficial discharge of deep groundwater. We estimate that He-4 in Guarani groundwater discharge accounts for about 20% of the assumed global flux from continental crust, and that other large aquifers may account for about 33%. Old groundwater ages suggest that He-4 in the Guarani aquifer accumulates over half- to one-million-year timescales. We conclude that He-4 degassing from the continents is regulated by groundwater discharge, rather than episodic tectonic events, and suggest that the assumed steady state between crustal production and degassing of He-4, and its resulting atmospheric residence time, should be re-examined.DOE, Office of Nuclear PhysicsNSFIAEA, Isotope Hydrol Sect, A-1400 Vienna, AustriaUniv Delaware, Dept Geol Sci, Newark, DE 19716 USAUniv Estadual Paulista, UNESP, CEA, BR-13506900 Rio Claro, SP, BrazilUniv Estadual Paulista, UNESP, LEBAC, BR-13506900 Rio Claro, SP, BrazilArgonne Natl Lab, Div Phys, Argonne, IL 60439 USAUniv Chicago, Dept Phys, Chicago, IL 60637 USAUniv Chicago, Enrico Fermi Inst, Chicago, IL 60637 USAUniv Chicago, Dept Geophys Sci, Chicago, IL 60637 USAUniv Bern, Inst Phys, CH-3012 Bern, SwitzerlandNatl Sci Fdn, Arlington, VA 22230 USAUniv Estadual Paulista, UNESP, CEA, BR-13506900 Rio Claro, SP, BrazilUniv Estadual Paulista, UNESP, LEBAC, BR-13506900 Rio Claro, SP, BrazilDOE, Office of Nuclear Physics: DE-AC02-06CH11357NSF: EAR-0651161