Before it was a notorious prison, housing the likes of Al Capone and George "Machine Gun" Kelly, Alcatraz was the site of a military fortress guarding San Francisco Bay.
Researchers have now discovered extensive tunnels, embankments and other remnants of a military fortress hidden beneath the floors of Alcatraz. Experts had thought these structures were completely destroyed long ago.
The U.S. military left the island during the Great Depression due to the costs of maintaining a fort and supplying personnel with food and water.
Researchers used ground-penetrating radar to determine the extent of the fortress complex beneath the former prison's recreation yard.
Images from ground-penetrating radar revealed the tunnels, embankments and other structures hidden beneath the prison for generations.
While researchers continue to explore the hidden world beneath Alcatraz, the island remains one of the most popular tourist destinations in the San Francisco Bay Area.