Photos: The Hidden Fortress Beneath Alcatraz

Military fort beneath Alcatraz

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.

Guarding the Bay Area

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 military leaves Alcatraz

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.

Ground-penetrating radar

Researchers used ground-penetrating radar to determine the extent of the fortress complex beneath the former prison's recreation yard.

Radar images

Images from ground-penetrating radar revealed the tunnels, embankments and other structures hidden beneath the prison for generations.

Alcatraz Island

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.

Marc Lallanilla has been a science writer and health editor at About.com and a producer with ABCNews.com. His freelance writing has appeared in the Los Angeles Times and TheWeek.com. Marc has a Master's degree in environmental planning from the University of California, Berkeley, and an undergraduate degree from the University of Texas at Austin.