SAN FRANCISCO (KTVU) - Alcatraz was America's most well- known federal prison for the nearly 30 years it operated off the coast of San Francisco.
But the notorious prison was hiding a secret that’s now come out more than 55 years after the federal penitentiary closed its doors.
Archaeologists and researchers from Chico State University, Binghamton University in New York, and Texas A&M have recently confirmed historians' long-time suspicion that Alcatraz was built over a Civil War-era military fortification with a series of tunnels and buildings under the prison’s recreation yard.
A study published last week in "Near Surface Geophysics" said archaeologists used ground-penetrating radar and laser scans as well as historical maps and photographs to locate the fully buried tunnels, structures and ammunition magazines.
"These remains are so well preserved, and so close to the surface," study author Timothy de Smet, an archaeologist at Binghampton University said. "They weren't erased from the island — they're right beneath your feet."
Archaeologists are now planning more studies in hopes of discovering what else lies just below the surface of “The Rock.”
Though Alcatraz is now famous for its role as a federal prison, its history as a holding place for criminals began before the Civil War. The U.S. Army first used the guardhouse’s basement cell room in 1859 to imprison soldiers who had committed crimes, according to the National Park Service.
By 1861, the government designated Fort Alcatraz as the official military prison for the entire Department of the Pacific, according to the park service.
The United States Department of Justice acquired the facilities in 1933 and opened it as a federal prison about a year later.
Between 1934 and 1963, Alcatraz held some of America’s most infamous gangsters, including George "Machine Gun" Kelly, Al Capone, Mickey Cohen and Whitey Bulger.
More than 1,500 men called Alcatraz home, and over the decades 36 different prisoners made 14 escape attempts, two men trying twice. Those who attempted to break free were either shot and killed or captured and sent back to their tiny cells.
In 1962, a year before the prison closed for good, inmates Frank Morris, John Anglin and his brother Clarenece Anglin escaped from Alcatraz by chiseling away with metal spoons at soggy concrete around air vents leading to a narrow utility corridor.
The prisoners were meticulous in their planning and execution of the escape, even leaving hand-made paper mache dummies with stolen hair on the heads in their beds to fool the roving prison guards.
Over many weeks, the inmates also constructed a raft out of stolen raincoats and on June 11, 1962 the three men slipped into the San Francisco Bay. Neither the men nor their bodies were ever found. They are officially listed as missing and presumed drowned.
But researchers say it’s unlikely that any of the would-be escapees or Morris and the Anglin brothers even knew about the underground tunnels and buried structures that the prison was built over. It wouldn’t have mattered anyway. The tunnels can't be physically reached except by radar.