Seven tons of human hair in Auschwitz. 8,985 human skulls in Choueng Ek. These are the stark realities that confront visitors to sites of genocide, torture and atrocity. The horrors of Nazi concentration camps in Poland or the Khmer Rouge’s Killing Fields in Cambodia can be taught in history books, but nothing comes close to the feeling of standing in the very place millions of people fell victim to evil.

"There's something about [it] that draws you into their reality in a way that's vivid and personal," said Sara Bloomfield, director of the United States Holocaust Memorial Museum in Washington DC. "And that just doesn't match any other form of learning... These places teach us that the unthinkable is thinkable," she said.

Preservation efforts in Poland are currently underway to ensure the protection of the decaying Auschwitz-Birkenau site and its victims' memories. This is good news for the country's tourism industry which sees a great deal of business from both foreign and local visitors to the Nazi death camp. Tourism, though, often brings with it some level of commercialization. In locations of atrocities around the world, tourists and tourism companies must ask themselves: when does tourism walk the line between education and exploitation?

Youk Chhang was just a child when he was separated from his family and forced into slave labour in the Killing Fields. Although he and his mother survived the Khmer Rhouge's reign, several of their relatives did not. Chhang now runs the Documentation Center of Cambodia, an organization that works to document the terrors of the Khmer Rouge.

"How can we learn from [this] history... so that it cannot be repeated, if we continue to fail to understand that the memory of those who have died cannot be commercialized?" he said in an email to BBC Travel. Sites of atrocities, he believes, "should not be sites to attract tourists, but places for them to learn".

Chhang fears that private companies coming into Cambodia are turning the Killing Fields into "Disneylands", as he told the Associated Press in 2006. He worries that resorts and casinos built near these sites may profit from the memory of genocide victims.

In Poland, too, some private tours are seen as controversial. Two stag party companies, Last Night of Freedom and Chillisauce, include a tour of Auschwitz in bachelor party packages that also offer bar crawls and visits to strip clubs. "Over 1 million people were systematically murdered at Auschwitz-Birkenau," Karen Pollock, head of the Holocaust Educational Trust, told The Sunday Times. "It seems entirely inappropriate to advertise a visit to the site for stag groups in this way, alongside nights of drinking and clubbing."

On the other side of the spectrum, even public museums sometimes worry about making money off of tragedies. In Washington, the Holocaust Memorial Museum has debated in the past, the issue of selling merchandise. "Originally...the feeling was that it would be inappropriate given the sacred nature of the topic," said Bloomfield, the director. "But then we realized, the museum gives [people] an interest in [the history of the Holocaust], so we need to give them a way to pursue that interest. So we needed to sell books in a bookshop as an extension of our educational mission."

Although balancing the need for tourism with the need for sensitivity can be difficult, some sites have done so successfully, says the International Coalition of Sites of Conscience. The ICSC is a worldwide network dedicated to remembering past struggles for justice at many different kinds of historical sites (not just those where tragedies occurred). "[T]he ideal", said deputy director Bix Gabriel, "is to create an experience that's sensitive to all the different kinds of relationships and stories that people may be bringing to that site."

In Chile, for instance, Villa Grimaldi, where political prisoners were tortured and detained under Augusto Pinochet's regime, survivors lead the tours of the site. Gabriel believes this to be a good model because it connects survivors with people who may not have a personal relationship with the site. The Villa, in Santiago's Parque por la Paz ("Peace Park"), strikes a balance, Gabriel notes, "between how to create a sort of sacred space that is sensitive to the needs of people who have come here to remember or to mourn, while also opening a space for a kind of exploration and curiosity and engagement and debate."

Similarly, in Cape Town's Robben Island, where Nelson Mandela was held prisoner for 18 years during apartheid in South Africa, tours of the maximum security prison are led by former political prisoners.

Bloomfield and Chhang both say that the most effective tours are the ones led by guides who have been well trained in the site's history. Bloomfield also emphasizes the importance of maintaining a sombre experience for the visitor. Auschwitz, for instance, limits the number of tourists during peak season.

Museums at sites of tragedies can also help contextualize them. In New York, the National September 11th Memorial and Museum is currently being built at Ground Zero. In Lithuania, the Museum of Genocide Victims houses a former KGB prison where dissidents were tortured, often before being executed.

For tourists to any site of atrocity, Bloomfield has this advice: "I think it is important to remember that you're entering a graveyard... You're going [to these sites] to learn from the past and achieve a deeper understanding of what took place there... but you're also going there to pay your respects to these people [who] died nameless and faceless."

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