This gasometer (Laclede Gas Company Pumping Station N) was an above ground natural gas storage facility. It's been out of service for a few years but overall everything was in pretty good shape. Climbing to the top of the dome, you could clearly smell gas and the closer you got to the top, you could hear a whistling noise as natural gas was rushing out of the hole in the top. Eh, that's probably ok.
EDIT 2013: This gasometer has been demolished.Gasometer: