To hold its political prisoners, North Korea built twelve massive labor prison camps in remote mountain locations that are sealed off from the rest of the North Korean society.
After their discovery by US spy satellites, six of the prison camps were closed and their inmates moved to the remaining six prison camps Chongjin, Hoeryong, Hwasong, Kaechon, Pukchang and Yodok.
North Koreans who enter these prison camps are almost never released. They are tortured (right), underfed and overworked until they die, are executed or used as guinea pigs in experiments to test biological and chemical weapons.
After they die, their corpses are eaten by rats, as evidenced by a witness' sketch:
(source: Committee for Human Rights in North Korea)
The rats are then eaten by the surviving prisoners, as evidenced by a witness' sketch:
(source: Committee for Human Rights in North Korea)
The six remaining North Korean prison camps today hold an estimated 200,000 prisoners. Several times as many are estimated to have perished since the prisons camps, which are calibrated to extract the greatest labor (or test results) per food input, were built.
Who ends up in these prison camps?
Anyone in North Korea who displeases the ruling regime, as well as up to three generations of the perpetrator's family. For example, if a man complains to a friend about Kim Jong-un, the man, as well as his wife, children and grandchildren (three generations) could be arrested and sent to these prison camps. And if the man complained about Kim Jong-un in public, everyone who heard the man complain and did not immediately report him to the authorities are also arrested and sent to the prison camps.
But many in the North Korean prisons camps didn't end up there because they made or heard a complaint. They are Christians who believe the True Gospel instead of Juche.