The Guinness Book of World Records has called the Pan-American Highway the world's "longest motorable road," but it's not really one single road. It's a network of different national highways that extend all the way from the Arctic Ocean to Tierra del Fuego, a distance of almost 14,000 miles. But a fun trek from pole to pole on the Panamericana remains agonizingly out of reach, thanks to a dense tropical forest on the Panama-Colombia border.

The "Panamericana" is the ultimate road trip.

Since Alaska's Dalton Highway opened in 1974, the Pan-American Highway has stretched all the way north to the oil fields of Prudhoe Bay, Alaska. Its southernmost terminus, meanwhile, is a gravel parking lot in Tierra del Fuego National Park, at the southern tip of Argentina. There a signboard helpfully notes, "Alaska: 17,848 km" for ambitious travelers heading north.

You can't get there from here.

But the Pan-American Highway doesn't quite cross all of those 17,848 kilometers (11,090 miles). In its span, it successfully passes plenty of obstacles: a ferry across the Strait of Magellan, a two-mile tunnel through the Andes on the Chile-Argentina border, a bridge over the Panama Canal, and the unforgiving tundra of the Yukon. But one obstruction has proved impassable: the Darién Gap where Central America meets South America.

Mind the Gap!

The Darién Gap is a remarkable study in contrasts, from the vast Atrato Swamp of the Colombian side to the mile-high peaks of the Panamanian side. The terrain does, however, have one striking commonality: It's very difficult to build roads through. The region is also dominated by an environmentally sensitive rainforest, home to several indigenous cultures, and provides a useful barrier in hindering drug trafficking and the spread of diseases like foot-and-mouth into North America. For this reason, multiple attempts to complete the Pan-American Highway through Darién have petered out.

Darién is the road less traveled—for good reason.

Today, most intercontinental travelers bypass the sixty-mile gap by boat or plane. But some adventurous souls still try the trek overland. A 1959 expedition crossed the gap in Jeeps and Land Rovers, and George Meegan's 1988 book The Longest Walk describes his seven-year stroll from Tierra del Fuego to Alaska, including his transit of Darién on foot. Today, the gap is even more dangerous than it was on Meegan's trip—a lawless wilderness where, even with a guide, it's not easy to steer clear of drug traffickers, bandits, and corrupt cops. Yaviza, Panama may look tantalizingly close to Turbo, Colombia on the map, but the last remaining 60 miles of the world's longest highway aren't getting completed any time soon.

Explore the world's oddities every week with Ken Jennings, and check out his book Maphead for more geography trivia.