This is the gas chamber. In the roof is the square shaft of blue light. This is where the Zyklon B was dropped.
I have always assumed that it killed relatively quickly. But when I saw this wall:
I knew I was wrong. To stand in this room where hundreds of thousands died was horrifying. The children were always on the top of the bodies. Heroes to the end. – the adults all assumed the air would be clearer higher and they tried to give these children a chance to live by holding the up close to the ceiling. It didn't work. The gas killed everyone – but not instantly. It took at twenty long horrific minutes. And the walls show it. Many of the children weren't with their mom or dad, but with strangers. Oh, the special hell that awaited all those who were silent.