The following testimony is from the affidavit of one Regina Bialek, prepared for the War Crimes Trial known as "The Belsen Trial", the trial of Josef Kramer and 44 other Nazis.
On 25th December, 1943, I was sick with typhus and was picked out at a selection made by Doctors Mengele and Tauber along with about 350 other women. I was made to undress and taken by lorry to a gas chamber. There were seven gas chambers at Auschwitz. This particular one was underground and the lorry was able to run down the slope and straight into the chamber. Here we were tipped unceremoniously on the floor. The room was about 12 yards square and small lights on the wall dimly illuminated it. When the room was full a hissing sound was heard coming from the centre point on the floor and gas came into the room. After what seemed about ten minutes some of the victims began to bite their hands and foam at the mouth and blood issued from their ears, eyes and mouth, and their faces went blue. I suffered from all these symptoms, together with a tight feeling at the throat. I was half conscious when my number was called out by Dr. Mengele and I was led from the chamber. I attribute my escape to the fact that the daughter of a friend of mine who was an Aryan and a doctor at Auschwitz had seen me being transported to the chamber and had told her mother, who immediately appealed to Dr. Mengele. Apparently he realised that as a political prisoner I was of more value alive than dead, and I was released.
- Source via Winston Smith Ministry of Truth