Workers Party of Korea monument, Pyongyang. Picture: Shutterstock
It houses more than 50,000 prisoners who are forced to live in subhuman conditions before being made to dig their own graves prior to execution.
It's also a place where women are regularly beaten and raped and where people are imprisoned just for being related to someone who has spoken out against the world's most secretive government.
And North Korea doesn't want you to know about it - or these top secret images which show the regime is expanding its biggest prison - camps 15 and 16 known as Yodok and kwanliso - to hold even more detainees.
Amnesty International has today released new satellite images showing the ongoing development of two of the country's largest political prison camps as proof the government is expanding its "vast infrastructure of repression".
The human rights organisation said the images clearly show new housing blocks, an expansion of production facilities, and continued tight security.
The latest images, taken in May this year, indicate a slight increase in kwanliso 16's population with new housing blocks visible.
The pictures, capturing housing being built, give a rare and possible insight into the living conditions detainees face.
Mining, logging and agriculture is also clearly visible in the images, which also show an expansion of an industrial area within kwanliso 16 and the areas where prisoners are forced to undertake hard labour.
Former detainees and officials who have spoken out or escaped from the camp reveal how inmates spend most of their time being forced to work in dangerous conditions.
Naturally security remains tight, with perimeter fences and security points clearly marked and movement restricted and controlled through secured entrance gates, guard towers and internal check points.
The analysis, along with newly released testimonies from former inmates and prison guards, is included in Amnesty International's latest briefing North Korea: Continued Investment in the Infrastructure of Repression.
It is only the tip of the iceberg of what goes on behind the horrific prison gates.
A former kwanliso 16 security official known as Mr Lee reveals exactly what takes place in the world's most secretive prison.
One former security official at Camp 16 in the 1980s until the mid 1990s, known as Mr Lee told Amnesty in an interview last month of just one brutal way guards executed prisoners.
He said detainees were forced to dig their own graves and were then killed with hammer blows to their necks. He also witnessed prison officers strangling detainees and then beating them to death with wooden sticks.