Tunnels were just discovered under the infamous high-security penitentiary of Alcatraz, but, surprisingly, they aren’t relics of prisoners’ attempts to escape. Archaeologists have long suspected that the prison was built over a Civil War-era military fortification, and these findings confirm it. Experts used ground-penetrating radar and terrestrial laser scans (to avoid disturbing the land while digging), as well as historical maps and photographs to find a series of tunnels, fully buried structures, and even ammunition magazines underneath Alcatraz’s prison yard.

According to Timothy de Smet, an archaeologist and co-author of the official study published last week in Near Surface Geophysics, “This really changes the picture of things,” he told PBS. “[The remnants of the fortifications] weren’t erased from the island — they are right beneath your feet.”

Alcatraz Island, now a busy tourist attraction in San Francisco Bay, served as a military coastal fortification in the 19th century during the Civil War, and started functioning as a federal prison for the most notorious gangsters in the country in the 1930s until 1963. It is now confirmed that the prison was built on top of the aforementioned military fortifications.

H/T: Insider