The Taum Sauk pumped storage plant is a power station in the St. Francois mountain region of Missouri, United States about south of St. Louis near Lesterville, Missouri, in Reynolds County. It is operated by Ameren Missouri. The pumped-storage hydroelectric plant was constructed from 1960–1962 and was designed to help meet daytime peak electric power demand. It began operation in 1963. Electrical generators are turned by water flowing from a reservoir on top of Proffit Mountain into a lower reservoir on the East Fork of the Black River. At night, excess electricity on the power grid is used to pump water back to the mountaintop. The Taum Sauk plant is an open-loop pure pumped operation: unlike some other pumped storage sites, there is no natural primary flow into the upper reservoir available for generation. It is therefore a net consumer of electricity; the laws of thermodynamics dictate that more power is used to pump the water up the mountain than is generated when it comes down. However, the plant is still economical to operate because the upper reservoir is refilled at night, when the electrical generation system is running at low-cost baseline capacity. This ability to store huge amounts of energy led its operator to call Taum Sauk "the biggest battery that we have". An unusual feature is the upper reservoir which is constructed on a flat surface, requiring a dam around the entire perimeter. On December 14, 2005, a catastrophic failure in the upper reservoir dam put the plant out of operation until it was rebuilt, recertified, and reopened on April 21, 2010. The new upper reservoir dam, rebuilt from the ground up, is the largest roller-compacted concrete dam in North America. Size and location The upper reservoir can hold about of water behind a wall nearly tall. It sits above the 450 MW hydroelectric plant, which gives it a greater head than that of Hoover Dam. The two are connected by a tunnel bored through the mountain. The Taum Sauk upper reservoir sits on ridge just below the summit of Proffit Mountain, not Taum Sauk Mountain, which is about to the east. It is visible from Route 21 north of Centerville and from Route N approaching Johnson's Shut-ins State Park from the south. Before the failure of the upper reservoir, visitors could drive to the top of Proffit Mountain and walk to an observation deck above the reservoir.