The controversial swimming pool. Signs nearby say that it was a water reservoir for the use of fire brigades, "built in the form of a swimming pool". To look at the thing, it's pretty obvious that it is a swimming pool. The problem is that holocaust deniers like to point at it and say that Auschwitz must have been a pretty friendly place to have something as nice as this. Most likely, it was both a water reservoir and a swimming pool, the use of which was limited to SS personnel and perhaps certain select prisoners who served their masters well.
Irregardless, Auschwitz One was not the main extermination camp. Many people died here, but not nearly at the level of Auschwitz-Birkenau.