It is one of the worst moments in history, which still horrifies to this day. During Hitler's brutal reign of Nazi Germany, more than six million Jews were killed.
But now new research has discovered that the Holocaust may well be even worse than previously thought.
Researchers at the United States Holocaust Memorial Museum have been documenting all of the Nazi concentration camps, ghettos, slave labour sites and killing factories which had been set up across Europe.
When they first started the project, the team expected to find about 7,000 camps and ghettos.
Shockingly, they discovered 42,500 camps across large swathes of German-controlled Europe. The researchers predict that up to 20 million people died or were imprisoned in the sites.
Speaking to the New York Times, Hartmut Berghoff, director of the institute, said: "The numbers are so much higher than what we originally thought.
"We knew before how horrible life in the camps and ghettos was but the numbers are unbelievable."
The figure includes 30,000 slave labour camps and 980 concentration camps. As well as 'killing centres' they included forced labour camps where prisoners made supplies for the war.
They also included sites called 'care' centres in which pregnant women were made to have abortions or their babies murdered shortly after birth.
The researchers have created a series of maps which present a grim view of life in wartime Europe. They show just how widespread the camps were, although most were centred in Germany and Poland.
Previous data has shown just the existence of individual camps on a fragmented basis.
But using data from 400 contributors, they have now documented the large scale operation for the first time.
When the project first started, the researchers expected to find about 7,000 Nazi camps and ghettos. But as the project has gone on, the numbers constantly increased to the current figure of 42,500.