Over 30 years ago it was revealed that Al Capone's secret vault in Chicago, Illinois was shockingly empty. The world-famous mobster took many secrets to his grave — apparently a trend that his followers also adhered to. But did you know there is another secret vault in Chicago, one that travelers can actually view and interact with (while also grabbing a bite to eat at a historic city building)? It's true, and its secrets are beginning to spill.

Harry Caray's Italian Steakhouse, 33 W. Kinzie St., is a historic building in the River North neighborhood of Chicago, just north of Downtown.

The restaurant is named after beloved Chicago icon Harry Caray, who was an announcer for both the Chicago White Sox and Chicago Cubs baseball teams during his storied career.

Classic dishes like hearty steaks, burgers, seafood, and chicken are all included on the menu alongside craft beers and cocktails.

But the real story of this Harry Caray's location is in its ties to the infamous mobster Frank Nitti, who once called the building home.

Nitti was a ruthless "enforcer" for mob boss Al Capone. In 1931, Nitti became the boss himself when Capone went to jail on tax evasion charges.

Nitti served as the acting boss of the Chicago Outfit gang until he died in 1943. And although he was still worth millions, that money was never found.

A look at the Harry Caray's building from the 1930s shows what it looked like when Nitti was living and conducting business there. While the area around it has changed drastically, 33 W. Kenzie St., is a piece of living history.

But after Harry Caray's moved into the building in 1987, all sorts of interesting secrets began to surface — like a heavy-duty safe. Though it had no money in it, customers sometimes leave a few dollars behind for the next person to find.

Then, in 1998, an electrician realized there was something behind a wall he was working on. It was soon discovered that a series of panels and doors led to an 85-foot room with 16-foot ceilings.

To reach the room, you must travel through a series of tunnels — which had allowed Nitti and his mobster friends to move about undetected.

Harry Caray's contacted the Chicago History Museum, which found that in old blueprints of the building there was indeed space created for a vault for Nitti.

During the Prohibition Era, many Chicago mobsters used underground tunnels and secret spaces to conduct business.

A camera through the wall discovered a wooden box that has yet to be retrieved. It's speculated — but not confirmed — that it could be part or all of the fortune Nitti left behind.

While the mystery of what's in the vault remains for one of the world's most infamous mobsters, visitors to Harry Caray's in River North can see some of Nitti's hiding places for themselves, along with historic documents, photographs, and other artifacts. The restaurant also includes Nitti's Supper Club and Speakeasy, an interactive show with musical performances and skits that recreates scenes and characters from Nitti's life. Hurry, these shows book far in advance!

Have you ever seen this incredible secret vault in Chicago, Illinois? What did you find most fascinating? Tell us in the comments! After you start planning your visit to Harry Caray's, be sure to check out this list of other must-see attractions in Downtown Chicago — or, if you're looking for other Midwestern mobster ties, take a look at this haunted Iowa hotel that has hosted Al Capone.

See more about Frank Nitti's incredible secret vault below!

Frank Nitti's Vault from Harry Caray's on Vimeo.