The significance of the Peña Blanca uranium deposits in the State of Chihuahua, Mexico as potential natural analogues for a nuclear waste repository in unsaturated, welded tuff was first recognized in the 1980s. In the 1970s, the Peña Blanca region was a major target of uranium exploration and exploitation by the Mexican government. Since then the Nopal I uranium deposit has been studied extensively by researchers in the U.S., Mexico, and Europe. The Nopal I deposit represents an environment similar to that of the proposed high-level radioactive waste repository at Yucca Mountain, Nevada in many ways. Both are located in semi-arid regions. Both are located in Tertiary rhyolitic tuffs overlying carbonate rocks that have been subjected to basin and range-style tectonic deformation. Both are located in a chemically oxidizing, unsaturated zone 200 m or more above the water table. The alteration of uraninite to secondary minerals at Nopal I may be similar to the alteration of uranium fuel rods in this type of environment. Investigations at Nopal I and in the surrounding Sierra Peña Blanca have included detailed outcrop mapping; analysis of drillcore, geophysical well logs and cuttings; hydrologic and isotopic studies of flow and transport; studies of mineral alteration; modeling; and performance assessment.