Q. On the ceiling of the main hall of Grand Central Terminal, there is a dark spot. I was told the restorers left it so that everyone could see how dirty the ceiling had been. Is that true? How big is it? Has this been done elsewhere?

A. It is true. 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.

Before the Metropolitan Transportation Authority started a two-year, nearly $200 million renovation of Grand Central in 1996, the entire ceiling had this unwanted finish, which came mostly from cigarette tar and nicotine.

Returning the ceiling to its former luster was a painstaking process. The restoration team tested a number of different cleaning agents, eventually applying the winning solution, called Simple Green, with cotton swabs.