Concentration camps. Crematoria. Gas chambers. Nazi medicine. Each brings to mind terrifying images of the universe created by Nazi Germany during its 12 short and violent years. All told, 11 million human beings were murdered by people who believed they were superior -- physically, genetically, culturally -- to other human beings. The Nazis and their allies built more than 10,000 camps across Europe to imprison their enemies and to implement their ruthless policy of radical racial eugenics that targeted 6 million Jews for extermination. That systematic murder was the Holocaust. But the Nazis also savagely murdered Gypsies, communists, Soviet POWs, Poles, the mentally ill, persons with genetic birth defects, the elderly, homosexuals, Jehovah's Witnesses, intellectuals, outspoken clergy, and nearly anyone else who was labled an enemy of the Third Reich. Their deaths reached 5 million.

Primo Levi, an Auschwitz survivor and author of numerous books on the Holocaust, made a prescient warning after the war: "It happened, therefore it can happen again. ... It can happen everywhere." His chilling words have proven correct in Tibet, Bosnia, Rwanda, Cambodia, and Iraq since the defeat of Nazi Germany by Allied and Soviet troops. I first read Levi in July 1994, as the last genocide of the 20th century, in Rwanda, ended. The genocide there and in Europe have been on my mind since. In 1997, I visited Rwanda. It wasn't until the summer 2000 that I had to chance to visit and document the world of terror created by the Nazis -- in the very camps they built, in the very rooms where they plotted the extermination of millions of men, women, and children.

Not everyone will have a chance to visit Holocaust sites to learn firsthand how human beings became mass murderers. For those who can't, this online guide provides a pictorial window into this tragic legacy. Some of the images, like the entrance to Auschwitz-Birkenau, are immediately familiar. They are global icons, known to all. Others, like the hot dog stand at Auschwitz and the subdivision inside the Flossenbürg camp in Germany, are not. They actually may shock those unfamiliar with the camps' current conditions. This is how the legacy appears in the more than half century since the end of World War II.