It's easy to think of our national parks as tame and sanitized places, full of cheery rangers, interpretive trails, and fifties-style cafeterias selling dubious burgers. But step a few yards from the neatly marked paths and you'll quickly realize how wild they are. Nowhere is this dichotomy more extreme than in Wyoming's Grand Teton National Park. Some 2.5 million people drive through each year, and most do little more than pull over to take an Ansel Adams-esque snapshot of the Tetons, which rise above the Snake River like a frozen, jagged tidal wave. Only a tiny fraction of visitors leave their cars behind and set off on foot, weaving daylong or weeklong hikes on the park's miles of backcountry trails. For this reason the landscape—a collage of glacial lakes and raw alpine wilderness—has hardly changed since the first intrepid fur trappers, the French voyageurs, stumbled upon the Snake River Valley in the late 18th century.

The real depth of this historical continuity struck me by chance, when I came across a dog-eared copy of a 19th-century magazine, Scribner's Monthly, in a New York bookstore. It was a firsthand account from the summer of 1872, when a group of American climbing novices took on the then unclimbed Grand Teton and—apparently—conquered it in a harrowing one-day ascent. Scribner's was the Adventure of the Gilded Age, and the gripping tale included vivid engravings of climbers dangling from precipices. The team was made up of the first geological surveyors ever to pass through Jackson Hole, and the two who claimed to have reached the summit sounded like extras from a Deadwood episode: The article's author was 40-year-old Nathaniel P. Langford, who had campaigned against outlaws in Montana and was the first (unpaid) superintendent of the new Yellowstone National Park; Captain James Stevenson, 31, was a decorated Civil War hero who had lived for years with the Blackfoot and Dakota Indians.

Reading about their ascent opened a window into a unique moment in park history. At precisely the same time—the summer of 1872—John Wesley Powell was descending the boat-swallowing rapids of the Grand Canyon's Colorado River, while over in Yosemite the wild-haired nature lover John Muir was scrambling up sheer cliffs with nothing but a few bits of stale bread in his pockets. It was part of an intense burst of interest in the West after the Civil War, and I began to think of those pioneers as the first American "adventure travelers." These characters had no specialized knowledge or training; they were simply inspired outdoorsmen who decided to plunge headfirst into the unknown, improvising and honing their skills along the way. Sure, things went awry—Powell's boats overturned; Muir was nearly brained in a number of falls; and one of the climbers on the Grand, Sidford Hamp, slid down an ice chute and nearly died—but they all made it through in the end. The backdrops for many of their journeys would eventually be designated national parks, and every summer like-minded adventurers head to them, inspired by the same freewheeling spirit as their historic counterparts.

Chief among these attractions is summiting the 13,770-foot Grand Teton, which has become a grail for first-time climbers: A couple of Jackson guide services give novices a two-day crash course in technical rock climbing, then escort them on a punishing overnight assault. About a thousand neophytes sign up annually—and despite much gnashing of teeth, blistering of feet, and tearing of palms, most of them make it. "Any fit person can manage it today," says Al Read, former president of Exum Mountain Guides."You just have to be in good aerobic shape."