The Mellon Arena's days are numbered after the Pittsburgh-Allegheny County Sports and Exhibition Authority voted Thursday to demolish the former home of the Penguins. Some had hoped to save the former Civic Arena, which featured the world's largest stainless steel retractable dome roof when it was completed in 1961. But the Penguins, who have moved to the new Consol Energy Center, control the development rights to the 28-acre site and plan to build office, residential and retail buildings to bolster the neighboring Hill District.