What horrors does Eliezer soon learn about the chimneys at Auschwitz?

They realized that the flames and smoke of Auschwitz would likely define the last moments of their lives.

"Over there, that's where they will take you. Over there will be your grave. You still don't understand? You sons of bitches. Don't you understand anything? You will be burned! Burned to a cinder! Turned into ashes!"

His anger changed into fury. We stood stunned, petrified. Could this be just a nightmare? An unimaginable nightmare?