For more than three years, North Korea has sealed its borders. People are banned from leaving or entering the country. Almost every foreigner who was inside has packed up and left. The world's most secretive and tyrannical state is now an information black hole. For months, three people inside North Korea have risked their lives to tell the BBC what is happening. What they reveal is shocking. Years of hard labour for those found watching foreign films and TV programmes, and execution for the ones who try to escape. Jean Mackenzie, the BBC's correspondent in South Korea, asks: is this a new dark age for North Korea?