Scientists are uncovering a hidden history lying beneath the prison of Alcatraz
It was America's most notorious prison.
Perched on a rocky outcrop in the middle of San Francisco Bay, from the 1930s to the 1960s the Alcatraz Federal Penitentiary was reserved for the "worst of the worst".
A who's who of the criminal underworld were incarcerated there: George "Machine Gun" Kelly, Mickey Cohen and Al Capone all spent time locked up in the tiny cells.
Today, the "escape-proof" jail is a much more accessible place: more than a million tourists visit each year.
But scientists say there's much more to "the Rock" than crime and punishment, and they have come to Alcatraz to investigate the hidden history that lies beneath the prison walls.
A team from Texas A&M University has gathered in the prison's recreation yard, where inmates would have spent as little as an hour a week away from the confines of the main block.
The researchers are slowly dragging a bright yellow cart along the ground, pulling it up and down in straight lines.
"That's ground-penetrating radar," says Prof Mark Everett.
"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."
Using this technique, the team has made a remarkable discovery: they have found the remains of a military fortress, which was thought to have been destroyed.
Standing in the middle of the yard, which is still enclosed by 6m-high (20ft) walls, Prof Everett points to a spot where he has found evidence of a subterranean tunnel system.
For the rest of the story: http://www.bbc.com/news/science-environment-26233170