A new study has found US Civil War-era tunnels and buildings buried beneath the famed Alcatraz island prison in San Francisco, California.
Historians had long-suspected that the notorious federal penitentiary had been constructed atop US military fortifications built in the 1800s.
A study published last week in Near Surface Geophysics describes the complex found beneath the prison yard.
The now-closed jail imprisoned some of the worst criminals in US history.
The study of the land, which is now controlled by the National Park Service, was conducted with ground-penetrating radar and terrestrial scans.
Beneath the prison's recreation yard, researchers discovered evidence of fully buried structures, ammunition magazines and tunnels.
"These remains are so well preserved, and so close to the surface," study author Timothy de Smet, an archaeologist at Binghamton University, told PBS.
"They weren't erased from the island - they're right beneath your feet."
BBC Science travelled with the archaeologist team to Alcatraz in 2014 to see firsthand how their research was conducted.
Armed with the new evidence, researchers are now planning to do more testing on the island known as The Rock.
The island was first claimed by the US government for military use after it was used to take control of California from Mexico in the 1840s.
During the US Civil War, Fort Alcatraz was used at the official military prison of the West Coast.
The first inmates from the federal prison system began arriving in the 1930s, and the last were moved out in 1963.
Some famed inmates on the high-security island include mobsters Al Capone, George "Machine Gun" Kelly, and Whitey Bulger.