Nearly 90 years have passed since South Terras, a commercial uranium mine in southwestern England, closed up shop. Miners and ore processors no longer come and go, but what about minerals rich in harmful radionuclides: What factors control whether they stay put at the abandoned mine or end up in rivers and groundwater? Scientists have chipped away at that research problem for years, and a new study has uncovered an unexpected environmental helper—arsenic. By using synchrotron-based X-ray methods to analyze soil samples collected at the mine, a team led by Claire L. Corkhill and Neil C. Hyatt of the University of Sheffield found that uranium combined with arsenic to form a highly water-insoluble mineral known as metazeunerite (Cu(UO2)2(AsO4)2·8H2O). The presence of the mineral, which had not been observed previously in sites contaminated with uranium, indicates that arsenic plays an active role in controlling uranium migration in the environment (npj Mater. Degrad.2017, DOI: 10.1038/s41529-017-0019-9). The study also turned up metatorbernite (Cu(UO2)2(PO4)2·8H2O) and suggests that the two minerals form a solid solution. The findings may be used to improve environmental models and strategies for remediating uranium contamination.