High above commuter’s heads, there is a small, black stain on the ceiling of Grand Central Station’s main concourse.
The distinctive rectangle (half on the teal background of the astrological mural, half on the beige bordering) is what the ceiling looked like before restoration efforts in the mid ’90s. The black residue was long thought to be caused from years of exposure to coal and diesel fumes, but spectroscopic analysis found the grime to be mainly from cigarette smoke. Restoration workers left the small patch behind as a reminder of how the ceiling looked before.
Interestingly, the stain is located just below the constellation of Cancer.
Image from indigotimbre