When you tell people about the Marathon des Sables, they generally have the same response: mouth agape, eyes bulging, "who the hell would want to do that?"
You start off by talking about the basics, the fact that it takes place in the Sahara Desert and that you have to run over 150 miles in six days.
Then you fill in the details, like that you have to carry everything you need to survive for that period except water.
You top it off by telling them about how temperatures regularly get into the hundreds and that you pay thousands of dollars to do it and use up a lot of vacation time.
Nine times out of ten the reaction is as described.
The answer to the question, who would want to do that, might surprise you.
I arrived in Morocco expecting to be confronted by a bunch of hard-core athletes who spent all their time talking about PowerBars and how much they benched.
The reality was quite different.
While all the contestants were certainly very fit, they came from all walks of life, had different interests, different body types. Some were running for the challenge, some for charity, others for the experience.
More than 800 people from 32 different countries entered the MDS this year, held from March 28 to April 7 —and there is already a waiting list for the 2011 race, it has become so popular.
So what is the appeal of the toughest footrace in the world?
Well, first of all, just that.
If you are looking to push yourself, you simply will not find a tougher competition than this one.
You effectively run a marathon a day for three days, then there's a super-long day — nearly 50 miles — and then you get one day off before running another marathon and then you run another 11 miles on the last day.
The terrain is brutal: you are trudging up sand dunes, sweating across salt-flats, struggling up mountains. There is sand everywhere, you get terrible blisters, there are no toilets or showers.
Yet, somehow, this is one of the most magical sporting events on Earth.
While the terrain might be tough to traverse, it is often stunningly beautiful: space and silence and sand as far as the eye can see. And the spirit of camaraderie that develops between all the different runners, notwithstanding age or gender or nationality, is simply amazing to watch.
I have never heard so much laughter, never seen so much hugging. I've also never seen so many people going to the bathroom — but that's another story.
It's inspiring, too, when you hear people's stories about what makes them run.
One of the U.S. contestants, Morgan Murri from Colorado, has recently started a charity, GO Leap, which raises money for socially disadvantaged children to go on Outward Bound-type programs.
He really got me thinking when he talked about the growth of "nature deficit disorder" in this country, a syndrome whereby kids who have no access and exposure to the wilderness are growing into dysfunctional adults.
"Kids not having that respect for the wilderness translates into not being respectful in society," he says. "One of the grounding principals of our country has been adventurism, and I think we've forgotten that."
Jeff Arricale, from Baltimore, ran to raise money for three different groups. One of them, John's Hopkin's Children's Center, treats two of his four children for a rare lung disease, called interstitial lung disease, for which there is no cure.
While he was just starting the longest stretch of the race, he received the news that his baby daughter was admitted to hospital. He was in an agonizing situation, being so far away from home at a time when his family needed him most, but the knowledge that he was raising money and awareness pushed him on.
"Every step is one step closer to Baltimore," he told us. "I've got 100,000 reasons to keep on going."
Not everyone runs for charity.
Kira Matukaitis, from Washington D.C., was motivated in part by her desire to live a "life less ordinary," and also by painful memories of middle school: "I was extremely overweight … And I got made fun of a lot … And I hope that anyone who may have been a fat kid, or still is a fat kid, knows that it can all be turned around. Maybe, I'm out there running for them."
For Ted Archer, the strongest runner on the American team, it was the challenge that made him enter.
He told ABC News, "I got an IM from this co-worker saying look at these crazy people. And I kind of wanted to be one of them."
One final wonderful thing about the Marathon des Sables is the emphasis on respect, for the environment and for fellow runners.
The organizers go to great lengths to keep the camps clean. Runners are told to burn their toilet paper and each bottle of water is labeled with the runners' number to stop them from littering along the way.
The race was started by a hugely charismatic Frenchman, Patrick Bauer, some 23 years ago.
Each year it gets larger, and more organization is required to manage the logistics (participants go through more than 70,000 bottles of water in one week). Notwithstanding this dramatic growth, the Marathon des Sables has maintained an intimate spirit of adventurism, camaraderie and internationalism.
For information on entering the Marathon des Sables, go to http://www.dreamchaserevents.com/mds/index.htm