Hidden history found beneath Alcatraz
J Thoendell stashed this in Tech
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.
Stashed in: Military!