Researchers from Texas A&M University have found the remains of a Civil War-era fortress underneath Alcatraz prison. The fortifications date as far back as the California gold rush and San Francisco’s earliest years as a boom town.

The fort was known to exist, thanks to old photographs of the site, but the details of the structure have remained murky until now. According to A&M’s Mark Everett, the structure was a “caponier,” a trench-like construction that would have been difficult for invading troops to capture without exposing themselves to significant losses: “It is a large structure that juts out into the bay and provides defensive cover. We have seen it in the old photographs but it has completely disappeared from present view.” Everett claims that much of the structure remains inaccessible beneath the prison structures, but that, between ground-penetrating radar and excavations on the prison yard, researchers should be able to get a good sense of what the structure looked like.

“Much like medical imaging would make a scan of the body,” said Everett, “we are making a scan of the ground under the rec yard. The tunnels would have been used for the fortifications. There would have been movement of man and ammunition; it would have been bomb proof and covered with earth so it would have been protected. We get signatures that indicate there is not only a tunnel, but magazine buildings too.”

The fort never fired a shot during the Civil War, but it did serve as a prison for Confederate sympathizers.

Alcatraz is, of course, best known for the prison that opened in 1933 and until 1963 housed some of the most notorious inmates in the U.S., including Al Capone and George “Machine Gun” Kelly. Known as “The Rock,” the island was originally just that—completely barren and lifeless—until human construction first brought dirt and other organic material. During its 29 years as a federal prison, 36 inmates attempted to flee on 14 different occasions, though none are thought to have been successful: 23 were caught, six were shot and killed, two drowned, and five are listed as “missing and presumed drowned.” The site is currently administered by the National Park Service and was named a National Historic Landmark in 1986.

Image via Wikimedia Commons