Workers in Rome have stumbled across a top-secret bunker once belonging to former Fascist leader Benito Mussolini, hidden underneath the historic Palazzo Venezia.

The discovery is the 12th such bunker and is said to have been the “most secret” of the former strongman’s hideouts, according to the Italian publication La Stampa.

And in what has become a tradition of sorts, the bunker will soon go on display for the public to tour and document, as has been done with other recently discovered Mussolini bunkers. City officials plan to install lighting, a touchscreen system and an air siren, meant to simulate the sounds of an impending air raid.

The nine-room compound was reportedly unearthed by city superintendent Anna Imponente and architect Carlo Serafini, who were inspecting a restoration project on the 15th century building that sits atop the bunker. The Palazzo Venezia currently houses a national museum and has been a historically significant structure for centuries, having been used by high ranking members of the Roman Catholic Church and other important figures over the years.

During their inspection, Serafini and Imponente noticed a tiny wooden hatch, which led down to the bunker nearly 50 feet beneath the earth.

“When we saw the concrete, it was all clear,” Serafini told the paper. "It’s the twelfth bunker of Rome—Benito Mussolini’s last bunker."

Although the bunker was never finished—there are holes in the wall meant for indoor plumbing and electricity—Serafini says the structure is so solid it would have likely held up under an assault from Allied forces.

"The walls rest on the foundations of an old tower, and are almost two meters thick in some places," Serafini told the paper. "It would have probably only been designed for Mussolini himself and one other person; more than likely his mistress, Claretta Petacci.”