Archeologists have confirmed a long-held suspicion of historians and say that famed Alcatraz prison was built over a Civil War-era military fortification.

Researchers have found a series of buildings and tunnels under the prison yard of Alcatraz Federal Penitentiary, which once held Prohibition-era gangster Al Capone, SFGate reports.

A study published in Near Surface Geophysics, an international geophysics journal, says archaeologists used ground-penetrating radar, terrestrial laser scans and historical maps and photographs.

They found fully buried structures, ammunition magazines and tunnels.

"This really changes the picture of things," study author Timothy de Smet, an archaeologist at Binghamton University, told PBS. "They weren't erased from the island — they are right beneath your feet."

Archeologists plan more study to discover what else lies just below the surface.

Jim Breeden of the Golden Gate National Parks Conservancy walks through the dungeons below the main cell house during a night tour in 2011. Archeologists have now found fully buried structures under the facility. (Eric Risberg/Associated Press)

Historians believe workers built over existing structures when the prison was built early in the 20th century.

Alcatraz first came to the attention of the U.S. government after it took control of California from Mexico in the 1840s. Its location in San Francisco Bay made it attractive for military fortification purposes.

During the Civil War, Fort Alcatraz was the military prison for the West Coast.

In the 1930s, the first federal prisoners — deemed by the media to be the worst of the worst — began arriving. The last inmate left in 1963.