Welcome back to WHY? In this installment, we visit the historic Grand Central Terminal—one of those New York City buildings that has a seemingly never-ending supply of stories and secrets.

On a visit to GCT earlier this month, we asked commuters coming through the main concourse if they knew about one of those secrets, hiding in plain sight.

On the constellation-filled ceiling of Grand Central's main concourse, there's a small dark patch at the edge where the mural and marble meet, in the northwest corner. That dark patch is an homage of sorts to this dirty ol' town—it was darkened through decades of exposure to a few pollutants, but mainly cigarette smoke. When the ceiling was finally cleaned in the mid-1990s, a decision was made to leave this small 9" x 18" rectangle untouched. The result is the city's most satisfying before & after.

The cleaning was part of a major restoration, and took around two years (from 1996 to 1998). Years ago the MTA's Marjorie Anders told Gothamist, "The area was left untreated by the cleaning solutions so that future preservationists will be able to tell if those solutions [used for cleaning] had any impact on the ceiling materials themselves. This is common practice among restoration artists."

She added that not everyone was on board with the idea, however. "It is a little known fact that many folks at the railroad did not want to leave a dirty patch," she said. "They wanted it all cleaned. Because I was frequently up on the scaffold with journalists during the restoration work, I got to know the crew doing the cleaning. They wanted to leave a 4-foot by 8-foot section uncleaned. But the General Superintendent of GCT would have none of it. In the end, on behalf of the two women who did most of the cleaning, Mary and Marina, I appealed to the President of the Railroad at the time who agreed to leave a much smaller patch." And so it was... and it hasn't been cleaned since.

During our recent visit, we met with George Monasterio, the director of Grand Central, who spoke more about the ceiling's dirty little secret. He explained that Simple Green and cotton rags were used, a cleaning solution that he says saved the railroad around $8 million, as it was the least expensive option and happened to work the best. Here's what else he had to say:

Actually, that isn't the ceiling's only dirty secret. While the mural was originally painted directly on to the ceiling, leaks caused damage that led to an entirely new (nearly identical) mural being placed over it in the 1940s. The whole thing was repainted... on insulating boards that contained asbestos. When the MTA renovated Grand Central in the 1990s, the authority decided not to restore the original mural because disturbing those boards could become a health hazard. So instead, the boards were cleaned. (These boards also contain a hole from a rocket—which you can read more about here.)

Fun fact: Both the original and the current ceiling have a big flaw. The design was mistakenly projected backwards, so nothing is where it truly belongs. The New York Times reported on this in 1913, writing, "A commuter from New Rochelle... pointed out wrathfully in a letter, west is east and east is west, and any one endeavoring to read directions by the zodiacal figures used in the decoration scheme would be entirely misled."

But that grimy patch made up of New York City dirt? It's perfect.

Do you want to know WHY? Send us your questions, which can be about anything from New Yorker behavior, to what a certain city agency does, to something you saw on the sidewalk that sparked your curiosity.