Hidden history found beneath Alcatraz

BBC

"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." . . .

. . . 

"The biggest potential discovery so far is at the south of the island, lying beneath the prison's parade ground.

" 'This is an area that is of most historical significance to the park service. It is a very important part of the fortifications," says Prof Everett.

" 'It is called a caponier, and it is a large structure that juts out into the bay and provides defensive cover. We have seen it in the old photographs but it has completely disappeared from present view.' " . . .

. . . Hat tip to Jeff Hayden; Texas