More than one million people, mostly Jews, were killed at Auschwitz-Birkenau, Nazi Germany's largest concentration camp in occupied Poland, between 1940 and 1945.

A group of survivors will take part in a ceremony marking the 75th year since the Soviet army liberated it from the Nazis, but that number is dwindling year after year.

The survivors' message is clear - that the world should not allow this to happen again.

Al Jazeera's Jonah Hull reports from the Auschwitz-Birkenau concentration camp, Poland.