In 2011, Maine State Prison launched a pioneering reform programme to scale back its use of solitary confinement. Bafta and Emmy-winning film-maker Dan Edge and his co-director Lauren Mucciolo were given unprecedented access to the solitary unit - and filmed there for more than three years. The result is an extraordinary and harrowing portrait of life in solitary - and a unique document of a radical and risky experiment to reform a prison.

The US is the world leader in solitary confinement. More than 80,000 American prisoners live in isolation, some have been there for years, even decades. Solitary is proven to cause mental illness, it is expensive, and it is condemned by many as torture. And yet for decades, it has been one of the central planks of the American criminal justice system.