Original sentence: Because of abandoned coal mines beneath the construction site, real concrete had to be pumped underground before construction could begin; more concrete was said to be used in the filling of the old mines than was used in the mall itself.DeBartolo Corporation of Youngstown, Ohio and the Pittsburgh-based U.Slag, a waste product of steel making, had for years been transported by rail cars from the mills of Pittsburgh to this once remote valley.Steel Corporation in the early 1970s.Steel industrial area, a huge slag pile once known as Brown's Dump.The pile grew until it became an artificial mountain, as hard as concrete and large enough to contain a mall (as well as many satellite stores).The planning and development of Century III Mall began as a collaboration between the Edward J.The site is a recycled former U.When the mall opened in 1979, it was the 3rd largest enclosed shopping center in the world