Once an isolationist communist state, over the last 20 years China has become the world's biggest exporter of consumer goods. But behind this apparent success story is a dark secret - millions of men and women locked up in prisons and forced into intensive manual labour.China has the biggest penal colony in the world - a top secret network of more than 1,000 slave labour prisons and camps known collectively as "The Laogai". And the use of the inmates of these prisons - in what some experts call "state sponsored slavery" - has been credited with contributing to the country's economic boom.In this episode, former inmates, many of whom were imprisoned for political or religious dissidence without trial, recount their daily struggles and suffering in the "dark and bitter" factories where sleep was a privilege. Charles Lee spent three years imprisoned for religious dissidence. He says: "For a year they tried to brainwash me, trying to force me to give up my practice of Falun Gong. They figured me out ... so they changed their strategy to force me to feel like a criminal ... because, according to their theory, a prisoner should be reformed through labour .... So they forced me to do slave labour."