There are hidden buildings and tunnels below Alcatraz's prison yard, study finds

A 1869 gun crew at Alcatraz with a 15-inch Rodman cannon. A 1869 gun crew at Alcatraz with a 15-inch Rodman cannon. Photo: National Park Service / GGNRA Photo: National Park Service / GGNRA Image 1 of / 44 Caption Close There are hidden buildings and tunnels below Alcatraz's prison yard, study finds 1 / 44 Back to Gallery

Sixty years too late for would-be escapees, a series of remarkable buildings and tunnels have been located under the prison yard of Alcatraz Federal Penitentiary.

A study published in "Near Surface Geophysics" details the steps archaeologists took to confirm the long-time suspicions of historians: that when Alcatraz prison was built in the early 20th century, it was constructed over the Civil War-era military fortification.

"These remains are so well preserved, and so close to the surface," study author Timothy de Smet, an archaeologist at Binghampton University, told PBS. "They weren't erased from the island — they're right beneath your feet."

So as not to disturb the land by digging, archaeologists used ground-penetrating radar and terrestrial laser scans in conjunction with historical maps and photographs. Below the prison recreation yard, they found fully buried structures, underground ammunition magazines and tunnels. Historians believe when the prison was built, workers elected to simply build over existing structures, rather than take the time and money to demolish them.

Alcatraz first came to the attention of the U.S. government after wresting control of California from Mexico in the 1840s. Because of its position in the middle of San Francisco Bay, it was viewed with interest as a military fortification. The island's natural topography was also appealing; the rock faces were used to create natural defenses. Brick buildings were constructed to house up to 200 soldiers and enough provisions to withstand a four-month siege.

By 1860, it was already being transformed into a prison. During the Civil War, Fort Alcatraz was the official military prison for the West Coast. In 1910, a new cell block was created for military prisoners. Two decades later, the Federal Bureau of Prisons took over operation of the facility. In August 1934, the first federal prisoners — deemed by the media to be the worst of the worst — began arriving. The last prisoner left in 1963.

While time has ground on above, the buried buildings have rested below, untouched.

Archaeologists are now planning more study to discover what else lies just below the surface.