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

SFGate reports 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 workers built over existing structures when the prison was built in the 20th century.

Alcatraz first came to the attention of the U.S. government after it wrested control of California from Mexico in the 1840s.

Its location in San Francisco Bay made it attractive for military fortification purposes.