THE MIGRATION OF PLUTONIUM is one of the gravest concerns for those planning long-term underground storage of nuclear waste. Plutonium, one of most toxic substances known, has a very low solubility in water and so it was once thought that this hazardous material would not move via groundwater. A new Livermore-Los Alamos study, however, suggests that plutonium might be making an aqueous journey aboard colloids (clays and zeolites). This is the chief explanation for the presence of plutonium in groundwater found 1.3 km away from a the scene of a Nevada nuclear test conducted 30 years before. The Department of Energy is now taking colloid transport into account in its formulation of a strategy for permanent waste storage. (Physics Today, April 1999.)