From tornado-ravaged towns to terrorist attack sites, there's something about tragedy that causes us to recoil in horror – but then run to the source to see it for ourselves.

Sites of major disasters – whether man-made or entirely natural – can be extraordinarily compelling. But why?

"A lot of people feel very sympathetic to victims of disasters, yet feel incapable of helping or supporting people who have suffered," says Dr. Michael Brein, a travel psychologist and author.

"To some extent, when you go to revisit the scene of a disaster, you're paying homage, expressing sympathy with what has happened."

The 9/11 Memorial in New York City is a striking example of this theory at work – within months of opening in 2011, upwards of one million visitors had visited the site of the worst terrorist attack in American history.

"Some of us feel, if we can revisit the scene of these disaster areas, we can maybe get a more genuine, more hands on, more sensory input of what has happened," says Dr. Brein.

In instances such as 9/11, he says, "we all go through a form of grieving – just as in the normal grieving process, visiting gives you some kind of completion, that you can now turn it around."

Still, visitors must be appropriately sensitive, says Kelly Schulz, spokesperson for the New Orleans Convention and Visitors Bureau. Schulz's family home of 30 years was devastated in the flooding that followed Hurricane Katrina, in 2005.

"I think the most important thing is to approach the situation and the local people with respect," says Schulz.

"Also know the right time to go," she says. "Stay out of the way until it is appropriate for visitors to travel to that site."

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