A stream that runs through Pittsburgh city limits, Nine Mile Run is surrounded by trees and wetlands, and fish and ducks make their home in and by the water. But just a few years ago, most of the run was a large slag dump and was choked with sewer runoff.
Pittsburgh's steel industry was the result of nearby geology. Nearby iron and coal seams along with river transport made the area a natural center for steelmaking, starting in the 1850s. For the Allied World War II effort, the mills supplied 95 million tons of steel. In the process of smelting iron ore for all this steel, the waste product left over is called slag, a glassy mixture of metallic oxides and impurities.
Various landfills received slag all over the area, but in 1922 the Duquesne Slag Company bought acreage along Nine Mile Run, built railroad track on the ridge above it, and dumped slag into the valley for 50 years. An estimated 200 million tons were dumped alongside the banks ten stories deep. Pittsburgh's old sewers would also back up and send runoff into the water on rainy days.
Starting in 2006, with the help of the US Army Corps of Engineers and the Nine Mile Run Watershed Association, the restoration of Nine Mile Run began and continues to this day. It is the largest restoration of its kind in an urban area in the country. Fish and ducks can be seen in the water. Wetlands naturally store and filter runoff. Trees in the meadow show vibrant fall foliage. One can walk the Nine Mile Run Trail all the way to where the run, once sodden with waste, meets the Monongahela River.
from Atlas Obscura http://atlasobscura.com.feedsportal.com/c/35387/f/665719/s/3eebcb9e/sc/8/l/0L0Satlasobscura0N0Cplaces0Cnine0Emile0Erun/story01.htm