Archaeologists Find Tunnels Beneath Alcatraz

The tunnels were not dug by prisoners, though — they’re from the Gold Rush era when the island was a military outpost.

San Francisco’s fascination with Alcatraz has always run deep, but we never realized how deep things actually ran beneath the former federal penitentiary on the island. Archaeologists and the National Park Service have just published a report confirming the existence of a network of underground tunnels beneath Alcatraz, over which the prison was built.  

The tunnels were not “discovered” through direct human exploration, but instead using ground‐penetrating radar and laser scans.

The full study on the tunnels was just published in the academic journal Near Surface Geophysics.“The fortification, with its underground ammunition magazines and tunnels, is important from a military history perspective,” the study says. “Remnants of buried structures including a ‘bombproof’ earthwork traverse and its underlying vaulted brick masonry tunnel and ventilation ducts were discovered to run east-west beneath the recreation yard.”

Image: National Park Service, Golden Gate National Recreation Area

Alcatraz was considered an important military outpost during the 1860s, as the U.S. and Mexico had just been at war over California, and Confederate forces were known to attack Union trade ships in the Pacific Ocean during the Civil War. Underground forts were the fashion at the time, as they were considered safer than above-ground structures in the event of a bombing.

Historians have long suspected that tunnels existed beneath Alcatraz, but until now have never had proof. “This really reinforces what several historians and archaeologists had long suspected,” the study’s author John Martini tells PBS Nova. “Up until this point, we had nothing to go on except for a few visible trace remains and maps — and a lot of suspicions.”

There’s one more fascinating historical tidbit disclosed in the study: In the 1840s, Alcatraz was known as White Island “due to copious amounts of guano left behind by the eponymous seabirds.” (Alcatraz is Spanish for “seabird.”)

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