The dark spot, a relic of an era when smoking indoors was routine, is located near the northwest corner, where the celestial blue of the ceiling mural meets the marble. It measures approximately nine by five inches.

The small, dark rectangular patch on the ceiling of Grand Central Terminal's Main Concourse—located near the constellation of Cancer—is a deliberate preservation of grime. During the massive restoration of the sky ceiling in the late 1990s, cleaning crews used a simple mixture of water and soap to remove decades of buildup. To show future generations the staggering difference before and after the cleaning, renovators left one brick-sized patch untouched. While many initially believed the black soot was the result of coal-fired trains that used the terminal until the mid-20th century, chemical analysis of the swabbed patch revealed a more surprising culprit: the grime was actually composed of 100 years of nicotine and tar from cigarette smoke. Before smoking was banned in public buildings, millions of commuters exhaled smoke that rose and adhered to the ceiling, slowly obscuring the vibrant turquoise and gold leaf mural until it was almost entirely black.