Yinzer here. The danger to Century III Mall wasn’t from the giant slag pile sitting right next to it, but the possibility of abandoned coal mine subsidence beneath it. The rumor was that there was more concrete pumped into the abandoned mines under the site than was used to build the mall. There are abandoned coal mines underneath SW Pennsylvania everywhere, and most of them were never documented.

Century III Mall was built by billionaire Edward De Bartolo Sr. who basically invented enclosed shopping centers, and was owner of the Pittsburgh Penguins and the SF 49ers. His daughter Denise owns the 49ers now. Malls were a good idea at the time, and back in the day I used to happily shop at South Hills Village. But they’re not a good idea anymore, so they are going away.