The mural above the concourse at Grand Central Terminal is one of the most gawked-at ceilings in New York City, but even daily commuters may have missed a peculiar feature. Tucked at the edge of the green and gold constellations is a rectangular black mark. The apparent blemish didn't get there by mistake: As Gothamist explains in its new series WHY?, it was left there by restorers when the ceiling was cleaned more than 20 years ago.

Prior to the Metropolitan Transportation Authority's renovation of Grand Central in the 1990s, the concourse was a lot dirtier. The station itself was constructed in Manhattan in the early 1900s, and the celestial scene that's on the ceiling today was painted there in the 1940s. It took only a few decades for tobacco smoke and other pollutants to stain the mural so badly that it needed to be restored.

Using Simple Green-brand cleaning solution and cotton rags, conservators spent two years scrubbing nearly every inch of the ceiling back to its former glory; the one part they skipped was a 9-inch-by-18-inch patch in the northwest corner. Sometimes, when doing a major cleaning project, preservationists will leave a small sample of the art or artifact untouched. If the cleaning products did any damage to the paint, the patch gives future preservationists something to compare it to. It also acts as a snapshot of what the mural looked like in its old condition.

To hear more about the mural and its dirty secret, watch the video from Gothamist below.

[h/t Gothamist]