JENN SHELTON'S biggest splash? Depends on how literal you want to get. There was the Rocky Raccoon ultramarathon, in February 2007, where she shrugged off a bloody nose and the swampy humidity of Huntsville, Texas, to run the fastest 100 miles ever recorded by a woman on dirt. Or there was the time, six months earlier, when she carbo-loaded beer and jalapeño pizza before the Vermont 100—and then left it all over the trail as she stumbled home in 29th place.

"I started running ultras to become a better person," Shelton told me when we first met. "I thought if you could run 100 miles you'd be in this Zen state. You'd be the Buddha, bringing peace and a smile to the world. It didn't work in my case. I'm the same old punk-ass as before, but there's always hope."

Those two extremes make the 25-year-old ex–surf bum from Virginia Beach the one to watch in what could be the ultra showdown of a lifetime: the Western States 100, taking place near Lake Tahoe this June 27–28. The women's field is so studded with all-stars, it's eclipsed even the dramatic return of Scott Jurek, who's returning after a four-year hiatus. Leading the women's pack is Nikki Kimball, a three-time Western States winner who, until she got injured in 2007, was undefeated in every trail race of any distance for seven years.

But which runner is Kimball most worried about?

"None of us has more potential to pop off a really amazing race than Jenn," says the 39-year-old runner. "No one has her raw leg speed." Problem is, you can never tell which Jenn Shelton will show up. With her dark braids and elfin build, she looks like pure innocence—notwithstanding the Bacardi 101 vampire bat she accidentally had tattooed on her right forearm. ("Truth in advertising," she said after learning that her bat was that bat.)