In my opinion, a particular problem with understanding the breadth and scope of Stalin’s mass murders, is the fact that he didn’t allow photographs to be taken, it was a punishable by death offense.
We have those horrible photographs of the German prisoner of war camps, and the crematoriums, and piles of bones, but Stalin did his murdering in secret. In fact, in the many instances where he actually had the soldiers kill whole groups of people (and pay them extra wages for doing so), he would then have those soldiers who had performed his killings then killed by others, and then those killed; and on and on it went. It’s actually pretty amazing we know what we do about those horrible times/crimes.
And then the fact of the Gulags located in the frozen tundra. There will come a day when that land finally unfreezes, when the bones of the millions who suffered and died there may finally be uncovered (what with global warming and all /s ) and the world will be amazed.
Google “Road of bones”