Hidden history found beneath Alcatraz

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....