Eastern State Penitentiary was once the most famous and expensive prison in the world, but stands today in ruin, a haunting world of crumbling cellblocks and empty guard towers.

Known for its grand architecture and strict discipline, this was the world's first true "penitentiary," a prison designed to inspire penitence, or true regret, in the hearts of convicts.

Its vaulted, sky-lit cells once held many of America's most notorious criminals, including bank robber "Slick Willie" Sutton and Al Capone.