Ameren operated a museum at the entrance gate highlighting the geologic and natural history of Missouri. The power plant was frequently visited by geology students because of a striking example of Precambrian/Cambrian unconformity in the rock layers exposed by the plant's construction. History Site selection In 1953, the Union Electric Company began to look for a site on which to build a new pumped storage plant. By 1958, company officials had decided to focus on sites with at least of vertical head, which would allow them to use a smaller storage reservoir. Union Electric hired Sverdrup-Parcel & Associates to evaluate potential sites in the St. Francois Mountains, which were chosen for their relative proximity to St. Louis (where Union Electric was based) and the favorably large elevation differentials (some approaching ). Sverdrup-Parcel & Associates intended to recommend Taum Sauk Mountain as the site, in part because it was the highest point in Missouri at , but strong political opposition to development of the scenic mountain (which was later incorporated into a state park) ruled it out. The nearby Proffit Mountain ended up being an excellent alternative thanks to its comparable elevation (, the 6th-highest point in Missouri), its close proximity to the East Fork of the Black River, and the reasonably short distance () to the existing power transmission grid. Construction Construction on Taum Sauk began in 1960. The construction team consisted of Fruin-Colnon Construction and the Utah Construction and Mining Company. It went into operation in 1963 with two reversible pump-turbine units that could each generate of power. It was not licensed by the US Federal Power Commission (the predecessor of the Federal Energy Regulatory Commission) because Union Electric contended that the FPC did not have jurisdiction over the non-navigable headwaters where the power station was constructed. In FPC v. Union Elec. Co., 381 U.S. 90 (1965), the United States Supreme Court held that the FPC did have jurisdiction and that Taum Sauk did require a FPC license. Although designed and built without federal supervision, a license was retroactively granted. At its completion, the Taum Sauk plant was by far the largest pumped storage plant in North America in a number of measures, and was considered a major milestone in the development of pumped storage technology. Its pump-turbines were the largest ever produced at the time of construction, it had an unusually high head (the highest in the US