Isolated in the San Francisco Bay and surrounded by steep cliff faces, Alcatraz Federal Penitentiary seemed like the most secure place to keep dangerous criminals in the mid-20th century. But it's recently come to light that every inmate on Alcatraz Island lived above a series of potential escape routes that predated the prison's construction, the San Francisco Chronicle reports.

In a new study published in the journal Near Surface Geophysics, archaeologists reported their discovery of structures and artifacts beneath the Alcatraz prison yard, including underground buildings, tunnels, and ammunition magazines. Guided by historical maps, documents, and photographs, they used laser scanning technology and ground-penetrating radar to locate the subterranean fortress close to the surface.

The site dates back to the mid-19th century, when Alcatraz Island was used for military purposes. The same natural features that would later make Alcatraz an appealing prison also made it an ideal coastal fortification. Enough brick buildings were built there to house 200 soldiers and enough food was shipped in to feed them for four months.

But the fortification wasn't used for its original purpose for very long. It was transformed into the West Coast's official military prison during the Civil War, and in the 1930s, the government turned it into a federal prison. Instead of tearing down the forts and tunnels leftover from its military days, workers left them intact and built over them to save money. Archaeologists plan to investigate the underground structures further without disturbing the historic site.

Alcatraz Prison closed in 1963, so the underground tunnels no longer pose a security problem. Today the island is part of the U.S. National Park Service and is a popular tourist attraction.

[h/t San Fransisco Chronicle]