Construction along Interstate Highway 99 during 2003 exposed sulfidic rock within a fresh roadcut on Bald Eagle Mountain at Skytop, near State College, in Centre County, central Pennsylvania. The cut exposed pyrite veins associated with an unmined, sandstone-hosted, zinc-lead deposit. Excavated rock was crushed and used locally as road base and fill. Within months, acidic (pH<3), metal-laden seeps and surface runoff from the crushed rock piles and roadcut raised concerns about surface-and groundwater contamination and prompted a halt in road construction and the beginning of costly remediation. The Pennsylvania Department of Transportation (PennDOT) and the Pennsylvania Department of Environmental Protection expanded their site investigations and monitoring in response to the acid-drainage problem to determine the extent of the environmental effects at Skytop and to develop long-term remediation strategies. In addition, those agencies sought information on the source of the acid drainage and on the processes that led to the formation of acid-rock drainage at Skytop to assist them in selecting appropriate remediation for Skytop and in avoiding similar problems in other areas. A conference on "Principles of Acid Pollution Control Along Highways" was convened by the Department of Geosciences of the Pennsylvania State University and the ClearWater Conservancy in December of 2004 to facilitate information exchange between scientists, engineers, environmental professionals, and the public. The conference highlighted a number of ongoing topical studies (geology, structure, conditions of formation of the mineral deposit, nature of the acid drainage problem, potential treatments) that are underway at the Pennsylvania State University and at Juniata College, as well as presentations on general principles of weathering, acid generation, and methods of prevention and treatment. Results of USGS studies on mineralogical controls on acid-rock drainage at Skytop were summarized at the conference and are presented in this report, along with other data for rock and water samples collected during site visits between May and September of 2004. This research was conducted as part of ongoing USGS studies of the geoenvironmental behavior of mineral deposits, the characterization of mine drainage, and modeling of near-surface processes. Cover photo Aerial view of the Skytop cut on January 6, 2005 looking north. Existing Route 322 in foreground. Future I99 roadbed runs along the base of the cut. Black plastic covers one of several areas of pyritic fill as a stopgap measure to minimize water infiltration. Photograph by Keith Brady.