Scientists have uncovered a series of 19th century tunnels underneath Alcatraz prison in San Francisco, California. The tunnels are part of a military fortress that used to stand where Alcatraz Federal Penitentiary was built on an island in the San Francisco Bay. The fortress was built over and all remnants were thought to have been destroyed, until now, the BBC reported.
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Using ground-penetrating technology, scientists from Texas A&M University found the fortress and tunnels under the ground in Alcatraz's recreation yard, where prison inmates would have spent a few minutes outdoors in the '30s. "(The tunnels) would have been used for the fortifications. 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," university professor Mark Everett told the BBC.

The team was able to use a ground-penetrating radar that scanned the ground under the yard, similar to how an X-ray scans the body, Everett told the BBC.

The island where Alcatraz was built is an integral part of American history. During the Civil War the island was used as a military installation, and later turned into an army prison at the beginning of the 20th century, the BBC reported.

The army prison laid the grounds for the island to be turned into a federal prison. But there was little to no evidence of the fortress left once the prison was built in 1915.

"What we don't really know is what exactly became of the fortifications, what state they are in and what is left of the cultural resources. And although it is not always desirable to excavate, with geophysics we can help people to know what is below the surface without actually disrupting it," Everett told the BBC.

Alcatraz prison is notorious for housing the "worst of the worst" criminals from the '30s to the '60s. Infamous criminals including Al Capone, Mickey Cohen and George "Machine Gun" Kelly were incarcerated on the "escape proof" island prison, the BBC reported.

Scientists hope to uncover evidence of other lost Alcatraz structures, including another potential structure underneath the prison's parade ground.

"We have a cultural landscape report that proposes to do an excavation on the parade ground," Jason Hagin, historical architect for the National Park Service, told the BBC. "It's our hope that in the future we can open up the trail and have an archaeological site that people can visit."