The Pan-American Highway runs sixteen thousand miles, from Anchorage to Tierra del Fuego, with one significant interruption: an expanse of rain forest along the border of Colombia and Panama. The road ends abruptly on the Panama side, just north of a national park, and picks up again as a dirt path, sixty miles southeast, in Colombia, in the floodplain of the enormous Atrato River. The region in between, which spans two coasts with jungles and mountains and a confounding web of rivers, is known locally as the Tapón del Darién—the Darién Plug—for its seeming impassability.

In English, it’s called the Darién Gap, the legacy of a nineteenth-century scramble to cut a seafaring channel from the Caribbean to the Pacific. The Prussian explorer Alexander von Humboldt speculated that the Darién isthmus harbored a river passage that need only be expanded to be navigable. In 1850, an Irish physician named Edward Cullen claimed to have walked such a passage without trouble, and his fraudulent assertion—supported by detailed phony maps—sparked a series of expeditions. Four years later, a twenty-seven-man team, led by Lieutenant Isaac Strain, of the United States Navy, set off to find Cullen’s mythical east-west passage. The team got lost within days and was forced to divide; seven men ultimately died. Strain refused to believe the indigenous Kuna who told him that he was going the wrong way, and months later he was found naked and sick, reduced to seventy-five pounds. He deemed Darién “utterly impracticable” for a canal, and engineers looked north to Panama City.

A century later, work stalled on the Darién link of the Pan-American Highway, and the gap came to mean something else: a breach in a road running north to south. American tourists arrived eager to hike it, and backpacking guides offered routes through the Serranía del Darién, the mountain range on the Colombia-Panama border. To automobile companies, the gap became an irresistible venue for publicity stunts. In 1961, a caravan of three red Chevrolet Corvairs took on “the world’s worst roadblock on the world’s greatest highway,” with support teams hacking trails and building bridges. Two of the cars managed to cross the gap; one was left to rust under a ceiba tree.

Scientists were equally attracted to the Darién. For millions of years, the isthmus has filtered the exchange of plants and animals between the Americas, and, as sea levels rose and fell, its mountains isolated populations, resulting in an extraordinary number of unique species. A fifth of its plants occur nowhere else. In 1981, after decades of intense study, the International Union for the Conservation of Nature declared that “thousands of species remain to be discovered.”

But in the late nineteen-nineties the left-wing Revolutionary Armed Forces of Colombia, or FARC, began fighting right-wing paramilitaries for control of the area. The only people to cross the gap with any frequency were combatants, illegal migrants, and drug gangs. Missionaries and orchid collectors were kidnapped, and, as the gap became synonymous with danger, science and tourism dried up. Los Katíos National Park, on the Colombian side, has been closed for years, owing to clashes among armed groups, who have seeded land mines there.

For a certain type of person, this is all very appealing. In 2003, Robert Young Pelton, the author of “The World’s Most Dangerous Places,” prepared for his own hike through the gap by e-mailing the FARC guerrillas and the region’s dominant paramilitary faction. He got no responses, but he was undeterred. At the last minute, he invited two young backpackers to come along, and all three ended up being kidnapped, when they stumbled on Colombian paramilitaries ambushing a small town. The fighters, armed with guns and machetes, killed four Kuna men, but after ten days they released Pelton and his companions unharmed. Pelton credited his wits for their survival. “It’s not really luck,” he told National Geographic News. “You’re in a certain mind-set when you’re kidnapped. You want to win the respect of your captors, so they drop their guard.”

Travel conditions in Colombia have improved since then. Demobilization of paramilitaries, waning FARC influence in the countryside, and better Army control of highways have made opportunistic roadblocks rare; kidnappings have decreased dramatically. In 2007, the government began advertising Colombia’s cultural and ecological wonders with the slogan “The only risk is wanting to stay.” Birders were among the first to take up the challenge, keen on adding the country’s seventy-six endemic species to their “life lists” of birds spotted. But even the promise of sooty-capped puffbirds and Tacarcuna wood quails has not enticed them into the roadless Darién Gap.

This March, I travelled with Sergio Tamayo, one of a very few guides offering tours in the gap. He began four years ago, when he was a twenty-four-year-old backpacker—“not a European-style backpacker but a Colombian-style backpacker,” he said, meaning that he worked as he travelled. He cut wood, mostly, and slept in a hammock. One day in San Francisco, a town on the Gulf of Urabá that its residents call San Pacho, Sergio heard some Medellín accents on the beach, and offered to take the visitors to an outlying island. He wrote up an itinerary, charged them twelve dollars apiece, and scrambled to find a boat.

Back in Medellín, he founded his own firm, Ecoaventurax, in an alcove of his mother’s apartment, and designed posters, T-shirts, and cell-phone charms with its logo. He got his chest tattooed with Maori-style designs and recruited customers on Facebook. He prefers people attracted to the gap despite, rather than because of, its dangers, but danger-seekers find him anyway. Two German men recently demanded that he take them to “where the guerrillas are,” and he said that he would take them close enough. The men painted their faces with jagua-fruit ink and posed for photographs in the woods. The majority of his clients, though, are, like him, young Colombians from modest backgrounds eager to experience parts of the country that have long been off limits.

Sergio has no politics to speak of—he was perhaps the only person in Colombia with no opinion about the death of Hugo Chávez—but he seems to have tapped into a broader strain of patriotism; last year’s ad campaign for Suzuki showed urban adventurers driving into the Colombian countryside, maps spread. Our itinerary included all the stops in Sergio’s weeklong gap tour, which he offers twelve times a year. The gap’s highlands remain under guerrilla control, so he takes visitors instead to the jungles and rivers that surround its few settlements: frontier towns, with varying degrees of lawlessness. We would travel northward along the Caribbean coast, through the clustered towns of San Pacho, Triganá, and Acandí, making our way to Capurganá and Sapzurro, at the border.

We started in an open boat from the port town of Turbo, crossing the Gulf of Urabá and heading west over the wide brown mouth of the Atrato River. Turbo is, for commercial purposes, Colombia’s last stop on the Pan-American Highway, and giant banana plantations and cattle ranches flank the road. Police and Army stations are everywhere, a legacy of clashes between paramilitaries and guerrillas. Billboards advertise shopping malls and model homes.

As we approached the western side of the gulf, our boat heaved and smacked along a shoreline probably little changed from when Vasco Nuñez de Balboa first saw it, in 1501: waves exploding against basalt boulders, a drab green curtain of forest broken up by the pink of guayacan trees in bloom. At our first stop, the town of Titumate, there was no dock, and men waded out to collect parcels. Three teen-age girls in tight sparkly tops—prostitutes—were being delivered in a fishing boat, and the men slung them over their shoulders like bags of cement. San Pacho, where we arrived twenty minutes later, did have a dock, and from it the town looked as though it had been built by pirates. Wooden houses jutted erratically from the hills, some of them shaped like ships and bearing tattered flags. Men and boys rode horses along the beach, waving up at Ruthie Laguado Zafra, the owner of a general store whose porch overlooked the sea.