Throughout the 20th century, Pittsburgh was the city of steel, at one point producing about one-third of the nation's steel output. By the 1970s, the city's steel industry had collapsed, plunging the region into tough economic times. And to add to the fiscal woes, the steel mills had left the city so polluted it was famously described as "hell with the lid off" (stories say that around lunch time, the smoke left the city looking more like 12 a.m. than 12 p.m.).

But you couldn't tell by the looks of the place now—Pittsburgh was named the most livable city in the U.S. in 2011, and one of the 20 places in the world to see in 2012 by National Geographic.

Check out these pictures of Pittsburgh during the steel years.

Tags: pittsburgh, pollution, steel
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