Auschwitz consited of three main camps and over 40 sub-camps. Auschwitz II or Birkenau was built in October 1941 and held more than 100,000 Jews; it was the main site of mass killings.

Birkenau was the epitome of Nazi efficiency. Gas chambers and crematoria capable of disposing of about 2,000 people a day were built there.

Jewish men, women and children were herded to Auschwitz in trains, like animals.

Those deemed too old or too weak to work were led to the 'showers' -- chambers where Zyklon B crystals, a pesticide used to kill rats, suffocated them. Then the bodies were burned in ovens designed specially for the purpose.

Rolling wagons slid in and out of the ovens so that corpses could be burned quickly.

The remains of the dead were put to industrial use.

By 1944, Auschwitz was killing 6,000 people a day.

The interior of a crematoria at Auschwitz.

Photograph: Scott Barbour/Getty Images