Treblinka extermination camp
In late July 1944, Soviet troops were getting close to Treblinka.  There were between 300 and 700 Jewish prisoners left at the camp.  The Nazi guards shot them all.  The Nazis then quickly destroyed the camp.  Before running away, they built a farmhouse for a watchman and sloughed over the ground to try to hide the evidence of their genocide.  Then they ran away from the camp.