Hidden tunnels below Alcatraz prison found

Archaeologists have used ground-penetrating radar and terrestrial laser scans to confirm that Alcatraz prison was built over a Civil War-era military structure.

Researchers found a series of buildings and tunnels under the prison yard of Alcatraz. (AAP) Source: AAP

Archaeologists have confirmed a long-time suspicion of historians and say that the famed Alcatraz prison in San Francisco 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 gangster 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 US government after it wrestled control of California from Mexico in the 1840s.

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