Under the Rock: Alcatraz's secret tunnels discovered

A lot of dark secrets about Alcatraz have been revealed over the years, and now a group of scientists has found one more buried under the former prison's walls.

Researchers from Texas A&M University, using ground-penetrating radar technology, discovered a network of tunnels underneath the San Francisco island prison. Experts had believed the tunnels were destroyed long ago. Professor Mark Everett explained the technology to BBC News.

From BBC News:

"The cart has a transmitter and a receiver — it sends an electromagnetic wave into the ground that then reflects off all the different structures underneath.

"Much like medical imaging would make a scan of the body, we are making a scan of the ground under the rec yard."

But why were there tunnels under the prison, often called "the Rock"? Was Al Capone trying to pull a "Shawshank"? Not so fast, Hollywood.

The tunnels were from the 19th century, when the island was used as a military fort. The researchers believe they also found magazine buildings where ammunition was stored as well as other structures.

The underground discovery can't be physically reached except by radar, Fox News reports.

Alcatraz began as a military fort, but it went on to become America's most infamous federal prison, housing the likes of Capone, George "Machine Gun Kelly" Barnes, Mickey Cohen and Robert Stroud. It shut down in 1963 due to concerns that rough Bay Area weather was affecting the integrity of the prison's walls. The escape of three prisoners in 1962 likely hastened the decision to close the rock.

Alcatraz has since gone on to become a popular tourist destination, drawing 1.3 million visitors a year.

Follow Mike Krumboltz on Twitter (@mikekrumboltz).