Pursuant to a congressional request, GAO provided information on abandoned hard rock mines on federal lands, focusing on the: (1) approximate number of such mines; (2) types of hazards the mines pose; and (3) approximate cost to reclaim the mines.

GAO found that: (1) the four major federal land managing agencies are each taking inventory of the abandoned mines on the lands they manage, but because the agencies do not use consistent methodologies to develop their estimates, there is no definitive inventory available; (2) the Forest Service has estimated of the number of abandoned mines on federal lands to be up to 25,000 sites; (3) nonfederal entities are also working to standardize terminology and guidelines to aid in future inventories; (4) abandoned hard rock mines can pose physical safety hazards, cause environmental degradation, and contaminate water; (5) the agencies use different factors to classify their sites for risk, and only two of the four agencies rank the severity of hazards; (6) nonfederal organizations have determined that 194,500 sites were generally safe, while 231,900 needed landscaping, 116,300 presented minor safety hazards, 14,900 could cause water contamination, and 50 threatened public safety and required complex cleanup; (7) the agencies have not completed the fieldwork needed to identify the number and types of problems on their sites; and (8) the Bureau of Mines believes that worst-case scenario costs could range between $4 billion and $35.3 billion and nonfederal organizations estimate that costs could exceed $70 billion, but no comprehensive cost estimate for reclaiming abandoned hard rock mines on federal lands exists.