The Nazi death camps built in Poland came to symbolise the depths to which Nazism would go to fulfill Hitler's dream of a 'master race'. The death camps were very different from the concentration camps found primarily in Nazi Germany. With the exception that some of the death camps had 'factory elements' to them, they were no less than places where mass murder was planned to take place. Such was the scale of these murders, that no truly accurate figure can be given. One accepted figure is 6 million murders in these camps. However, the Nazis did all they could to destroy what records existed as to the numbers they knew they had murdered. The advancing Russian army found that the Nazis had the time to destroy documents before they fled. The Russians found many documents but the Cold War meant that they were not available to historians generally.
The most infamous of the death camps were Auschwitz-Birkenau where as many as 2 million may have been murdered; Sobibor where about 250,000 were murdered; Treblinka where 725,000 may have been murdered; Chelmno where 600,000 were murdered; Belzec where 600,000 were murdered and Majdanek where 235,000 were murdered. Another camp was found in north-west Poland at Stutthof where 67,000 were murdered. All of these camps were in Poland with four of them near the Russian border (Belzec, Majdanek, Sobibor and Treblinka) while Auschwitz-Birkenau was in the south-west of Poland while Chelmno was in central west Poland.
There is no doubt that the Allies knew about the death camps long before the Russians liberated Majdanek. Some prisoners did escape from the camps and told the Polish resistance movement exactly what was going on in the camps and this information was sent to London to the Polish Government in exile who accordingly informed the Allies.