OSWIECIM, Poland - There is something terribly wrong here. Scribbles of clouds cruise across a blue sky that hovers over neat, rectangular red-brick buildings. A loitering breeze wimples green leaves, exposing their gray undersides, and beyond, fields swim in yellow and purple flowers. These bright colors are unnerving. This should be only in black and white, like a grainy old newsreel.

The Poles call this Oswiecim, but the Nazis called it Auschwitz, and the name still leaves a metallic taste in your mouth. Here, between May 1940 and January 1945, death was a way of life, established and organized along the principle of absolute evil. At least 1.1 million people, mostly Jews, were killed here with industrialized efficiency - usually within an hour after they arrived. Here is the biggest cemetery in the world, but there are no gravestones.

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A visit to Auschwitz is a jarring contrast to the usual sightseeing destinations. Auschwitz leaves a wake that you bob around in for days and weeks. Nevertheless, the most effective genocide machine in human history is now a UNESCO World Heritage site that attracted 1.4 million visitors last year, nearly three times the total of just a decade ago.

"Mass tourism here is a good thing," says Andrew Klosinski, a Polish guide for Odysseys Unlimited, the operator of our small group tour. "The more people who come here, the better. There are no longer survivors to tell their stories. Teenagers today have only secondhand information, and it is so easy to forget." Eyeing a swelling crowd pushing beneath the iron entrance gates, he adds: "The horrors of this place are unbelievable. The only thing worse would be to forget them."

Odysseys Unlimited, which is based in Newton, Mass., runs about 20 trips a year to Eastern Europe. Each has a full measure of visits to museums, cathedrals, castles, and battlefields, but Auschwitz is part of every itinerary.

"Many guests tell us that originally they were somewhat nervous or apprehensive about visiting Auschwitz, but afterwards they greatly appreciated the opportunity to see the memorial and museum for themselves; to be able to bear witness at the site of one of mankind's greatest atrocities," said Sue Bonchi, Odysseys' vice president of marketing. "Actually walking the grounds, seeing it up close, brings their understanding of the Holocaust to a new level."

The full name of the site is the Auschwitz-Birkenau Memorial, which includes the original camp and a satellite camp two miles away that was called Birkenau. There are 75 original buildings, including barracks and the remains of gas chambers and crematoria. Near the entrance is the handsome villa that was home to Rudolf Hoess, Auschwitz's first commandant, his wife, and three children. Railroad tracks, leading to nowhere, cut through the camps like a wound.

"Auschwitz would have been impossible without the railroads," explains Anna Ren, a Polish Jew who led our group of 22 Americans through the camps. "This site was chosen for two reasons: It was relatively isolated from outside eyes, and it had excellent rail connections to all of Europe to bring people here quickly and efficiently."

At the end of the rail line there is a cattle car, similar to the ones that carried Anne Frank and millions of others here. Pebbles have been placed on the car, which by ancient Jewish tradition are gestures of remembrance and respect.

Ren recites grim statistics as we pass mounds of victims' hair (seven tons), and piles of shoes (40,000 pairs) and artificial limbs (12,000). There are heaps of dolls and toys, tons of hairbrushes, and a mountain of suitcases brought by people who were deceived into thinking they were going to resettlement. Auschwitz scratches the eyes.

Ren speaks without intonation, her lips tight like a ventriloquist's, with a professional detachment. She might be giving tomorrow's weather forecast. But there's a trace of bitterness when she says that 90 percent of the guards escaped punishment.

There are thousands of visitors here, but nearly everyone is either silent or talking in low syllables of sorrow. There is a sadness so deep it's almost tangible. A woman in our group says that, before she was born, her parents were here and survived, but her grandparents and siblings she never knew did not. "If my parents could stand this place for a year, I can stand it for an hour," she says, pursing lips to stop the tears.

Ren says she has been doing these tours five days a week for eight years and sometimes needs to take a week off to convalesce mentally.

Some travelers in Eastern Europe refuse to go to places such as Auschwitz, but trying to avoid the Holocaust here is like trying to avoid the American Revolution in Philadelphia.

There were constant reminders during a 17-day tour of Poland, Hungary, Austria, Slovakia, and the Czech Republic.

In Krakow, Poland, there's a new museum housed in the World War II factory that was run by Oskar Schindler and made famous in Schindler's List, Steven Spielberg's 1993 Academy Award-winning movie about the German businessman who saved the lives of 1,100 Polish Jews.

The Schindler Factory Museum, which opened in 2010, uses the story of how Schindler shielded Jews by employing them in his factory as a springboard to tell the story of Krakow before and during the Nazi occupation from 1939 to 1945. There are period artifacts, photographs, and documents as well as personal testimonies from Krakow residents. There is film footage shot from the windows of a tram during the occupation. Schindler's office includes his original desk, telephone, lamps, and a huge map of Europe on the wall.

In Prague, the walls of the Pinkas Synagogue, founded in 1479, are inscribed with the names of 80,000 Czech Jews who were killed at Auschwitz. On whitewashed walls, the names appear in red and their dates of birth in black. They are listed alphabetically by family surname, with as many as 10 individuals in a single family. Among the victims were the grandparents of Madeleine Albright, the former U.S. secretary of state, who visited here in 1997.

Upstairs there is an exhibition of art done by children who were held at the nearby Terezin concentration camp and later killed at Auschwitz.

In Warsaw, we visited the ghetto that was the site of the 1943 Jewish uprising, which was brutally suppressed by the occupying Nazis, but only after three months of fighting.

In Budapest, there is a monument along the bank of the Danube consisting of 60 pairs of iron shoes, commemorating thousands of Jews who were shot and dumped into the river. The victims were ordered to leave their shoes, which were valuable.

In Bratislava, we stood at the statue of Raoul Wallenberg, the Swedish businessman who rescued Jews by giving them fake passports and bribing Nazi bureaucrats.