Archaeologists have confirmed a long-time suspicion of historians: the 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 Al Capone.

A study published in “Near Surface Geophysics” says archaeologists used ground-penetrating radar and terrestrial laser scans and historical maps and photographs. They found fully buried structures, ammunition magazines, and tunnels.

Historians believe that when the prison was built in the 20th century, workers chose to build over existing structures.

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

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