A new discovery is giving Alcatraz — the famous prison that served as an involuntary home to some of the “worst of the worst” criminals –even more of a backstory to add to its fascinating history.

Researchers have found history buried under the prison on Alcatraz Island, which sits in the San Francisco Bay. (Justin Sullivan/Getty Images)

Researchers have found history buried under the prison on Alcatraz Island, which sits in the San Francisco Bay. (Getty Images/Justin Sullivan)

While the metal bar-lined hallways might have their own story to tell, researchers at Texas A&M University, Chico State University and the National Park Service found something underneath “the Rock” — a hidden fortress.

Before becoming a prison that housed some of the country’s most famous convicts like Al Capone and George “Machine Gun” Kelly, Alcatraz Island was the site of a lighthouse, a military fort and military prison. It was previously suspected that the island might contain hidden tunnels connected to the Civil War. Using ground-penetrating radar, the team is bringing up the truth.

Alcatraz in the 1800s (Image source: NPS)

Alcatraz in the 1800s (Image source: NPS)

“We know from records and drawings that Alcatraz, at one time called Fortress Alcatraz, has been heavily fortified,” Mark Everett, Texas A&M professor of geology and geophysics, told the university’s GeoSciences News. But this fortification was thought to be destroyed.

The team used ground-penetrating radar to pick up on any structures that might be underneath the prison. (Image source: YouTube)

The team used ground-penetrating radar to pick up on any structures that might be underneath the prison. (Image source: YouTube)

“[The tunnels] would have been used for the fortifications. There would have been movement of man and ammunition; it would have been bombproof and covered with earth so it would have been protected,” Everett told BBC News. ”We get signatures that indicate there is not only a tunnel, but magazine buildings too.”

Among the more notable structures is a caponier, a type of fortification structure, which the team thinks is buried under ground.

Fort Alcatraz pictured in 1910. (Image source: NPS)

Fort Alcatraz pictured in 1910. (Image source: U.S. National Park Service)

“We have a cultural landscape report that proposes to do an excavation on the parade ground,” Jason Hagin, a historical architect with the U.S. National Park Service, told the BBC. “It’s our hope that in the future we can open up the trail and have an archaeological site that people can visit.”

Watch this video about the team’s research on the historical island:

In addition to finding the fortification in what appears to be relatively good shape, researchers are also excited by what it could be made from.

“Originally, the fortifications were earthen — they are constructed of dirt —  but parts of them had concrete over them to reinforce them,” Dr. Tanya Wattenburg Komas with Chico State University told the BBC, explaining that this system could be made with the oldest concrete in the U.S. “The interesting thing is we weren’t even making cement in the U.S. at that time.

“That probably came as cement in barrels from Europe. To find it on the top of a mid-19th Century battery is very exciting.”

(H/T: Daily Mail)