Mobile colloids-suspended particles in the submicrometre size range-are known to occur naturally in ground water(1,2) and have the potential to enhance transport of non-soluble contaminants through sorption(3). The possible implications of this transport mechanism are of particular concern in the context of radionuclide transport. Significant quantities of the element plutonium have been introduced into the environment as a result of nuclear weapons testing and production, and nuclear power-plant accidents. Moreover, many countries anticipate storing nuclear waste underground. It has been argued that plutonium introduced into the subsurface environment is relatively immobile owing to its low solubility in ground water(4) and strong sorption onto rocks(5). Nonetheless, colloid-facilitated transport of radionuclides has been implicated in field observations(6,7), but unequivocal evidence of subsurface transport is lacking(3,8,9). Moreover, colloid filtration models predict transport over a limited distance resulting in a discrepancy between observed and modelled behaviour(3). Here we report that the radionuclides observed in groundwater samples from aquifers at the Nevada Test Site, where hundreds of underground nuclear tests were conducted, are associated with the colloidal fraction of the ground water. The Pu-240/Pu-239 isotope ratio of the samples establishes that an underground nuclear test 1.3 km north of the sample site is the origin of the plutonium. We argue that colloidal groundwater migration must have played an important role in transporting the plutonium, Models that either predict limited transport or do not allow for colloid-facilitated transport may thus significantly underestimate the extent of radionuclide migration.