UNDERGROUND nuclear explosions produce trace amounts of distinctive but ephemeral radionuclide gases. In the context of monitoring a comprehensive test ban treaty, the detection of these gases within the territory of a signatory, during a challenge inspection(1-5), may indicate the occurrence of a clandestine nuclear event. Here we report the results of an experiment simulating a well-contained underground nuclear explosion, undertaken to test the ability of natural gas-transport professes to move highly dilute and rapidly decaying radionuclides to the surface. We find that trace gases are transported to the surface within periods of weeks to a year, by how along faults and fractures driven by barometric pressure variations. Both our observations and related simulations exhibit a chromatographic behaviour, with gases of higher atomic mass and lower diffusivity reaching the surface more rapidly. For a 1-kilotonne nuclear test under conditions identical to those of our experiment, we predict that short-lived Xe-133 and Ar-37 would be detectable, respectively, about 50 and 80 days after the detonation. Our results indicate that radionuclide sampling along natural faults and fractures, as a forensic tool, can be an extremely sensitive way to detect nearby underground nuclear explosions that do not fracture the surface.