Oct. 3, 2013 -- Think you have what it takes to finish the longest, toughest, most outrageous horse race on earth? The Mongol Derby is a bone-crushing sprint through more than 600 miles of rugged Mongolian Steppe. Thirty competitors from all around the world raced through hailstorms, downpours and oppressive heat in August all on the backs of 800 wild Mongol horses. Only 18 made it to the finish line.
This year was the race's fifth installment and for the first time, two young women were locked in battle for first place -- an unlikely rivalry that ended with a shocking twist.
"Nightline" traveled to the derby's starting line in Mongolia's vast wilderness, where it is believed man rode a horse for the first time 4,000 years ago, and followed this group of extreme adventurers, who paid up to $10,000 for the privilege to push their physical and mental limits.
Among this year's competitors were three Americans: Devan Horn, a 20-year-old hotshot from Houston, Lynne Gilbert, a lifelong equestrian -- and at 55, one of the oldest participants -- and Tom Burk, a 23-year-old Texan college grad, who somewhat ironically recently entered landed a job as a risk management analyst.
"I think if there's one thing I've learned about working in risk… it's that you got to take it when you can," Burk said.
The night before the race, derby organizer Katy Willings raised a glass of the Mongolian national cocktail airag, fermented mare's milk, for a toast. After the toast, Burk accepted a challenge from a native Mongolian -- a wrestling match. Unfortunately, Burk injured his shoulder and had to see the medic.
Day One started at 6 a.m. as riders made their final preparations. They are allowed just 11 pounds of gear, so luxuries like extra toilet paper are left behind.
At the starting line, Horn was raring to go. Gilbert came with a plan. She and her new friend from France, Sandra Fretelliere, formed an alliance to finish as a team. Meanwhile, Burk, with his arm wrapped up, was still dealing with the aftermath of the previous night's antics.
"Torn ACM or something, a bunch of ligaments, looks like I have a bone spur sticking out of my shoulder," he said, before saddling up.
With a final blessing from a Buddhist monk, the competitors were off on one of the wild Mongol horses, legendary beasts revered for centuries for their power and endurance, but feared for their unpredictable nature.
Horn was the first to reach the first horse station checkpoint and arrived alone. Being first meant she got first pick of a fresh horse.
"He's skinny, he's short, he's got nice legs and his feet are worn down so that means he rides a lot, so he's probably not crazy," she said, looking over one of the steeds before choosing one.
She checked the coordinates for the next station and was off.
Eventually, the rest of the pack started to arrive at the first station too, including Lara Prior-Palmer, who said she was turned off by Horn's Texas bravado.
"Some really competitive Texans and 'I'm here to win and if I don't finish in seven days, I'm not going home,'" she said.
Instead, the 19-year-old Brit had a different philosophy to surviving the race.
"You got to have sort of a 'anything goes attitude,' that you can deal with any situation because you have no idea what's going to happen next and I think that's what's so fun about it," Prior-Palmer said.
Burk reached the first checkpoint walking his horse in -- that shoulder of his had become a real problem.
"It wasn't the most comfortable ride, but I guess I will get used to it," he said. "More pain pills."
But then there was more bad news. At each station, a veterinarian performs a thorough check-up on each rider's horse and Burk's horse's heart rate was too high to allow him to continue, which meant he had to stay put until the horse's heart rate returns to normal, costing him time.
Meanwhile, by the end of Day One, Horn had made it to the third station -- 24 miles ahead of anyone else. By Day Two, Prior-Palmer made it to the second horse station.
By Day Two, the Gilbert-Fretelliere duo too had made significant progress, but when they reached the 8:30 p.m. cutoff for riding, they found themselves nowhere near a horse station. Their choices were to sleep rough or hope for nomadic hospitality from a Mongolian family.
"They say this is something you should do when you do the race--go with a family that doesn't know you're coming," Gilbert said. "I'm not accustomed to just walking into somebody's house and expecting them to feed me and give me a place to sleep."
Then came reports that Horn fell off and lost her horse, which can be a huge penalty, but miraculously she found it in the Mongolian wilderness and got back on. In addition, she was feeling ill.
"At like two o'clock I got the worst stomach flu in the history of man," Horn said. "I was puking and having diarrhea and it was terrible… And I couldn't get off my horse."
Her misfortune allowed Prior-Palmer to close the gap on her lead, putting them in a practical dead heat at a critical point in the race.
Meanwhile, Burk was still clinging on. Despite the searing pain in his shoulder, he made it over 200 miles and to nine horse stations. But as the gauntlet slogged into Day Five, he was dealt a derby fiasco.
"So I'm walking again, my horse's cinch was slipping a bit so I got off to tighten it up and then I tried to get back on, it bucked me off and took off back to the beginning of the herd," he said.
He had no choice but to turn back.
"I think I might be dead," Burk said. "I was getting back on and really, really twisted my shoulder. It kind of was up, and back. And I mean, it's been a lot more painful than I've been letting on."
At his breaking point, Burk was the sixth rider to quit. The others who were still in the race were wishing they weren't. A stomach bug that had hit Horn was wreaking havoc on the other riders. After Gilbert came down with diarrhea, her partner, Fretelliere said she was disappointed to continue alone.
"I wanted to finish together," she said. "It's really sad, but let's see the positive. She can still catch up to me."
But it wasn't easy. One after the other, the Mongol Derby took riders down until only 18 remained. Of those, the battle for first place was down to just two, Horn and Prior-Palmer, and it was neck to neck between the two, both riding at a punishing pace, until something shocking happened.
Tune into "Nightline" TONIGHT at 12:35 a.m. ET to find out what happens.