Photos: The Hidden Fortress Beneath Alcatraz
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Military fort beneath AlcatrazBefore 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.
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Guarding the Bay AreaResearchers 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.
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The military leaves AlcatrazThe 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.
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Ground-penetrating radarResearchers used ground-penetrating radar to determine the extent of the fortress complex beneath the former prison's recreation yard.
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Radar imagesImages from ground-penetrating radar revealed the tunnels, embankments and other structures hidden beneath the prison for generations.
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Alcatraz IslandWhile 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.