Far from the brutalist posturing of Pyongyang, the machinery that really maintains Kim Jong-uns control of North Korea is the brutality of the gulags.

Channel 4 News has highlighted areas of some the the main prison camps. Click on the gulag symbol to see maps and the facilities they include.

As Channel 4 News explores North Korea: Uncovered, the testimony of those who have "defected" from the state, and satellite imagery, shed ever newer light on life inside the massive concentration camps.

Channel 4 News Asia Correspondent John Sparks spoke to Jeong Kwang-il, a defector who had been detained at Yodok, otherwise known as Kwan-li-so No. 15.

The smell of rotting bodies was terrible. We had to use pitch forks to drag them out. Jeong Kwang-il

A kwan-li-so is a political penal-labour colony, for those who are deemed to have criticised the government.

Mr Jeong told how in the winter the prisoners were made to cut down trees, and those who died during the work were "piled up".

"We had to pile them up in the shed, and you could hear the dying ones wailing through the night, groaning with pain," he said.

"When April came we had to go into the shed and clear it up. The smell of rotting bodies was terrible. We had to use pitch forks to drag them out."

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Yodok is perhaps the best known of North Korea's gulags, and it is thought that at least 50,000 prisoners are held there.

According to the United Nations special papporteur for human rights in North Korea, thousands of people held at Yodok are there by reason of "guilt by association", and many do not know the reasons for their imprisonment.

The camp is divided into villages, says a report by US Committee for Human Rights in North Korea, and is surrounded by three-metre high fences with watch towers every kilometre.

Areas inside the camp include Pyongchang-ri - a punishment or detention area, and Kouek - a secluded killing area. There are also re-revolutionising zones, where prisoners are re-educated, and sometimes released back into North Korean society.

Many "defectors", who usualy head into China and then on to South Korea - a journey that can ultimately take many months of planning and money-raising - have been released from the camps but believe they will continue to be watched closely by the North Korean security service.

Spilling ink

An Hyuk was detained at Yodok between 1987 and 1989. He told the Committee for Human Rights in North Korea that prisoners, some of whom were detained for "crimes" such as spilling ink or failing to adequately dust photographs of Kim Il-Sung, were subjected to sleep deprivation and forced to sit motionless for days.

Work included a construction project where he was forced to break ice and wade into frozen streams to gather stones - a task that led to dozens of people dying from exposure.

Hospitals, as can be found on the interactive map above, were reported by detainees to be no better than places where the injured were left to recover, or die.

Kaechon is another kwan-li-so, around 30 miles long and 19 miles wide. Work at the labour camp included mining, clothing manufacture, farming and livestock raising.

At the camp, guards would couple model prisoners together, and they would be allowed to start families. Shin Dong-hyuk was a prisoner born to such a union.

Read more from Asia Correspondent John Sparks: Why the official version of North Korean life is a carefully constructed lie

He described how children would be beaten by guards, and placed in "schools" which were, in fact, child labour squads.

Over the Taedong river from Kaechon is Bukchang - believed to be home to around 27,000 prisoners, including the families of "wrong-doers".

Bukchang is considered to be a less severe prison camp. Civilians run some sections of the camp, and prisoners are able to earn "tokens". Families were reported to be allowed to live together, and privileged prisoners were allowed to marry people of their choice and have children.

Subsequent reports have suggested that large parts of Bukchang may have been dismantled, whilst other parts combined with Kaechon.

A fourth kwan-li-so, located at Hoeryong, was reported to have closed in mid-2012.

'Make a good person'

North Korea is also home to another kind of prison camp - the kyo-hwa-so - a long-term prison-labour facility for those convicted of crimes.

Unlike the vast kwan-li-so facilities, these are often compounds of buildings surrounded by guard towers. Kyo-hwa-so translates as a "place to make a good person through education"; in reality the education is hard labour.

Former prisoners interviewed by the human rights commission described how detainees would be forced to do hard, dangerous work, on insufficient food. There were no prisoners' privileges and death rates were reported to be high.

If you listen to North Korean state television, which Channel 4 News has been streaming with a translation, you will often hear tales of the generosity of North Korea's leader Kim Jong-un - be it giving a family a house, or opening a new sports facility.

But it is perhaps through the testimony of those fleeing the state's "death camps" that we can see the true nature of North Korea's leadership.