I grew up in Braddock Hills and there was at least one open mine that we were aware of as kids. I think it was in the woods behind houses that line Brinton Road. We used to call the area the Coal Fields when we were kids (late '60s, early '70s). Got curious about the facts and found this link that has a bit more detail ... Also, my recollection is that mine subsidence caused major problems on the "new" Swissvale High School back in the late 70s.