If St. Charles County had a "bad place," it was the concrete husk that stood off Highway 94 on what is now a firing range for the sheriff's department. Its name, when it was alive, was Water Treatment Plant No. 2. The main structures included an office building, two lime storage towers, and a water tower. The federal government began building the plant in 1941 to purify water used to make TNT at the Weldon Spring Ordnance Works. The plant was "born" upon its completion in October, 1942. It pumped between 20 and 38 million gallons of water per day throughout World War II. The plant "died" when the government shut it down in 1946.
But it did not stay dead. The plant became a legendary hangout for young people. Rumors spread that Satanic cults performed occult rituals and animal sacrifices within its cavernous chambers. The lure of mystery and danger attracted more thrill-seeking teens year after year. Some dubbed the plant the "Echo Dome." Others called it the "Aqua Dome." In time, the nicknames merged into one – the Equadome.
http://www.stateofhorror.com/equadome.html