I've visited Auschwitz-Birkenau twice. The thing that made the biggest impression on me was the scale of the camp and the design for efficiency in killing. The place is huge.
Below is a picture of the front gate that the rail cars went through.
A picture of the rail sorting area which we've all seen in movies like Sophie's Choice and Schindler's List.
And a picture of what was labeled The Last Walk at the camp. This is the path that those that didn't make the cut at the sorting area walked down towards the gas chambers where they would often queue up wait their turn.
The whole place is chilling. Blood run cold - chilling.