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.
Alcatraz is known for being one of the most infamous prisons in United States history. From the 1930s to 1960s it housed the likes of Mickey Cohen and Al Capone and some people recall the brief period known as The Alcatraz Occupation, where Native Americans controlled the island for nineteen months, from November 20, 1969, to June 11, 1971. Today, its mostly known as one of the most popular tourist destinations in the San Francisco Bay Area.
But the BBC reports that a team of scientists from Texas A&M University discovered traces of the long-forgotten Alcatraz military base hidden underneath the prison a fortress that historians believed had been destroyed years before.
Using ground-penetrating radar, the scientists have scanned the recreation yard, which is still enclosed by 20-foot walls. There the team discovered evidence of a subterranean tunnel system.
[The tunnels] would have been used for the fortifications, explains Professor Mark Everett. There would have been movement of man and ammunition; it would have been bomb proof and covered with earth so it would have been protected, he explains. We get signatures that indicate there is not only a tunnel, but magazine buildings too.
So why was this structure there in the first place?
I think most people know that in 1848 gold was discovered in California, and before that time San Francisco was really a very small town, explains Jason Hagen, the historical architect for the National Park Service, which cares for the island.
Images from ground-penetrating radar revealed the tunnels, embankments and other structures hidden beneath the prison for generations.
But once gold was discovered here, San Francisco became a very important port for the country and for the west coast, and so protecting it really was the point of building the fortress of Alcatraz.
Eventually the American Civil War broke out and Alcatraz became a military installation for the protection of the city. However, there was no offensive action happening in San Francisco, so it was first turned into a military prison, and then into the federal penitentiary. During those periods, it was believed that the old fortress was destroyed or built over. Evidently, those assumptions were wrong.
Scientists, architects and other officials are continuing to study the area with radar technology to determine if excavations are necessary.
Researchers used ground-penetrating radar to determine the extent of the fortress complex beneath the former prisons recreation yard.
See more amazing photos and get more on the story at the BBC website.