Grand Central Terminal is Gotham’s Beaux Arts jewel, a monument to transportation that avoided demolition in the 1970s (unlike poor Penn Station a decade earlier) and ennobles the lives of the 700,000 people who use it daily. It’s so vast, with nearly a century of history under its tracks, that it holds many secrets, as Metro-North Railroad spokesman Dan Brucker said. Here’s a selection:

1. The dark patch 
Look up at the ceiling: In the northwest corner, you’ll see a little square black patch. Now imagine extending that color across the entire constellation that’s painted on the ceiling. That’s what was there before Grand Central Terminal was dramatically restored in the 1990s. That little black patch was left as a reminder of the bad old days. And what exactly does that black patch consist of? Decades of dirt? Try again. It was the result of decades of smoking in the terminal. That’s old nicotine and tobacco residue that was preserved, and it’s a testament to how dramatic this restoration was.   
2. The rocket hole
Toward the center of the ceiling, above the constellation Pisces, you’ll notice a little hole. You’d never see it if you didn’t know to look for it. But the hole is a curious legacy of the space race. In 1957, the Russians put Sputnik into orbit and the U.S. was keen on selling the public on the importance of staying ahead. In a curious bit of showmanship, a Redstone rocket was brought in for display at Grand Central Terminal that same year. But some genius didn’t think to measure whether it would fit in the concourse. Well, surprise, it could not, and it was rammed in, leaving a hole in the ceiling that’s still there.

3. The clock
The clock atop the information booth in Grand Central Terminal is not only a beautiful work of art, it may be worth more than $10 million, according to auction house estimates. That’s because of the four opal faces on the clock.

4. Wrong departure times
Every single time shown on the departure boards is wrong. If that train to, say Croton-Harmon, is set to leave at 11:20, it’s flat-out lying to you. It’s leaving at 11:21. All trains leave a minute later than indicated on the departure boards. The reason is the safety and comfort of commuters who are making a mad dash to catch the trains.  

5. The new twin staircase
The grand marble staircase on the eastern side of the terminal was built in the 1990s to resemble the one that dates to 1913. Original plans did in fact call for the construction of eastern stairs. But there is one crucial difference between both stairs. The new set is an inch smaller than its original twin across the concourse, and the reason was to make it clear to future generations that the staircases were not built at the same time.

6. The whispering gallery
Just outside of the Oyster Bar restaurant is a vault covered in Guastavino tile. If you stand in one corner of the vault and say something, your voice is telegraphed perfectly to someone standing clear across the other side, dozens of feet away.  

7. The acorns
All around Grand Central, you see what appears to be a “squashed pineapple,” as Brucker put it. They are actually acorns, a Vanderbilt family symbol.

8. The backward universe
The universe as depicted on the ceiling is beautiful, but it’s also backward, a fact discovered by a commuter almost as soon as Grand Central opened.  The problem caused no small amount of consternation to the Vanderbilt family, but then they came up with a brilliant idea. They’d claim the error was indeed intentional, and say it was meant to depict God’s view of the universe from somewhere up above.                                              

9. Recycling bins
For years, one of the best secrets of Grand Central — and, really, it was an open secret — was that you could wander onto the platforms, plunge your arms into the recycling bins, and walk away with free copies of all the day’s newspapers. One newspaper in particular, The New York Times, was not happy about this at all, and so, in 2001, had the recycling bins redesigned so that commuters could not get their grubby mitts on the free newsprint, which they were doing to the tune of about a ton every morning. 

10. Wonders below
Grand Central is full of secret spots that the public may well never see: Well below the main concourse is a room with ancient machinery that was targeted by German saboteurs during World World II. In this room, there’s even a red button that could halt train traffic above. The area is so deep that it cuts into bedrock.  Farther north, under the Waldorf-Astoria, you can find a platform, an elevator and an old rail car that Brucker said were used by Franklin D. Roosevelt, who tried to keep his paralysis from the public.