Alcatraz Island, located in the San Francisco Bay, served as a military fortress and prison during the Civil War. Reuters

Long before Alcatraz prison housed some of America’s most notorious criminals, the San Francisco Bay island was the site of a Civil War-era military base whose walls and tunnels were buried and forgotten – until now. Using ground-penetrating radar, a team of scientists have found a labyrinth of secret, 19th century tunnels hidden underneath the old prison rec yard.

By sending electromagnetic waves into the ground, researchers from Texas A&M University were able to reflect all the different structures that lay underneath and map the 150-year-old tunnel system, the BBC reports. Their scans revealed old fortifications and even magazine buildings dating back to long before the construction of the main prison building in 1915.

"From 1850 to 1907 was the era of Fortress Alcatraz," Mark Everett, an A&M professor of geology and geophysics who is part of the team studying the secret Alcatraz tunnels, told the Houston Chronicle. “We know the embankments and such are under the rec yard, but we don't know their exact configuration. Our job now is to try to build a picture of what's actually there.”

During the American Civil War, Alcatraz Island served as a military base that held West Coast Confederate sympathizers. The defense structures were built starting in the mid-1850s and included barracks and gun batteries. While much of the country was embroiled in bloody battle, the Alcatraz fortress remained quiet with not a single defense shot fired.

Researchers were surprised to find that some of the tunnels, built mostly from earth, were reinforced wtih concrete, which wasn't yet available in the U.S.

“The interesting thing is we weren't even making cement in the US at that time," Tanya Wattenburg Komas, from California State University Chico and director of the Concrete Preservation Institute, told The BBC. "That probably came as cement in barrels from Europe. To find it on the top of a mid-19th Century battery is very exciting."

Much of the old 19th-century fortress remains inaccessible, buried beneath existing prison buildings. Still, archaeologists hope to start excavating the secret Alcatraz tunnels.

"The fortification is something that is really important to the historical significance of the island itself," Jason Hagin, an architect for the National Park Service, which oversees the island, told The BBC. "In that sense, it is really important to give people a sense of the early part of the island's history."