The New York Times said Sunday the World War II records revealed such a widespread presence of such sites that Holocaust researchers had to double-check their figures to make sure they were accurate.
"The numbers are so much higher than what we originally thought," said Hartmut Berghoff, director of the German Historical Institute in Washington told The Times.
"We knew before how horrible life in the camps and ghettos was, but the (new) numbers are unbelievable."
The review performed by the United States Holocaust Memorial Museum cataloged 42,500 camps and ghettos in Germany, Russia and other areas of Europe occupied by the Nazis before and during the war. Researchers said they expected the total to be around 7,000 when they began their exhaustive project.
They ranged from the well-known concentration camps down to small work sites and even brothels set up to service German soldiers.
The largest on the list was the infamous Warsaw Ghetto in Poland where 50,000 Jews lived. Scored were killed over the years, which culminated in a bloody uprising that was crushed by the Germans.
Between 15 million and 20 million people are believed to have perished at the sites, the researchers estimated.