Alcatraz Island in San Francisco Bay has a long history as a military and federal prison. But before the Rock became the Rock, the site was home to Fort Alcatraz, a military installation commissioned to protect the then-burgeoning city of San Francisco. That fort, and most signs of it, are long gone. But as Katie Dowd at SFGate.com reports, archaeologists recently located hidden structures below the concrete prison that show a glimpse of what was.

Because of its historical importance—Alcatraz Island is part of the Golden Gate National Recreation Area and administered by the National Park Service—archaeologists couldn’t just dig up the mostly cement prison yard to see what was under it. Instead, researchers used non-invasive ground-penetrating radar and laser scans to locate to see if they could find anything still lying beneath. Afterward, they compared their results to old maps, historical documents and photographs to help them interpret their data. All together, they believe they’ve located an ammunition magazine, a “bombproof” earthwork traverse as well as brick and masonry tunnels. The finds are revealed in the journal Near Surface Geophysics.

“This really changes the picture of things,” lead author Timothy de Smet, an archaeologist at Binghamton University, tells Katherine J. Wu at Nova. “These remains are so well preserved, and so close to the surface. They weren’t erased from the island—they’re right beneath your feet.”

There’s a reason the fort at Alcatraz didn’t last. Adam Brinklow at Curbed San Francisco reports that construction of a brick and masonry fort on the island began in 1853, but even before it was completed, powerful new artillery made that style of construction obsolete. In 1870, the military began a project to upgrade the island to sturdier earthwork fortifications, but that plan was not completed. Instead, during the Civil War and late 1800s, the military began using Alcatraz as a prison and quarantine zone for soldiers with tropical diseases. Its reputation as a prison fort grew from there, and in 1907, the U.S. officially designated it a military prison. The military went about constructing the massive concrete cellblock it’s become known for—the world’s largest reinforced concrete building at the time of its completion in 1912—burying the remains of the old fort and earthworks beneath.

Maintaining the island prison was a costly endeavor, not to mention Alcatraz’s harsh conditions were bad PR. In 1933, the military transferred the prison to the U.S. Bureau of Prisons where for the next 30 years it served as a segregation unit for the most difficult prisoners housed in federal custody. That’s the era that made the Rock legendary in popular culture. Due to the expenses of running it, it closed its doors for good in 1963.

The new archaeological research shines a light on the long-lost history of the island before Al Capone, Machine Gun Kelly and the “Birdman” Robert Stroud made it infamous. “This really reinforces what several historians and archaeologists had long suspected,” co-author and Alcatraz historian John Martini tells Wu. “Up until this point, we had nothing to go on except for a few visible trace remains and maps—and a lot of suspicion.”

In the article, the researchers suggest this type of non-invasive research can help archaeologists investigate other sensitive places, too, so as they put it they can “figuratively rather than literally dig up an otherwise inaccessible but fascinating past.”