Illustration by Brown Bird Design

The Amazon in South America is the longest river in the world—or so says a recent survey by the government of Brazil, which has a vested interest in staying a few steps ahead of the Nile. The Amazon has a vast jungle basin, breathtaking waterfalls, and the world's largest river dolphins, but it might be more remarkable for something it doesn't have: a single bridge. That's right, the Amazon is the world's longest river not crossed by any bridges.

Welcome to the jungle! We don't have a bridge.

The Amazon runs over 4,300 miles from its headwaters in the Andes to its massive delta on the Atlantic. While the 25 million people who live on or near its banks span Brazil, Peru, and Colombia, living in sprawling metropolitan cities and tiny tribal villages, all have one thing in common: to cross to the other side, they'll need to jump in a boat or hop on a ferry. No road crosses the Amazon. It divides an entire continent nearly in two.

Bridges in the Amazon would work great… until January.

For most of its length, the Amazon isn't anywhere close to too wide to bridge—in the dry season. But during the rainy season, the river rises thirty feet, and crossings that were once three miles wide can balloon to thirty miles in a matter of weeks. The soft sediment that makes up the river bank is constantly eroding, and the river is often full of debris, including floating vegetation islands called matupás, which can measure up to 10 square acres. It's a civil engineer's worst nightmare.

Where we're going, we don't need roads.

But the real reason for the lack of bridges is simply this: the Amazon Basin has very few roads for bridges to connect. The dense rainforest is sparsely populated outside of a few large cities, and the river itself is the main highway for those traveling through the region. Macapá, on the north shore of the Amazon delta, is a city of half a million people, but there's not a single road to connect it to the rest of Brazil. If you rent a car there, the only direction to drive it is north toward French Guiana.

The basin's first bridge is good for commuters but bad for trees.

For years, ferry traffic between Manaus, Brazil and its sister town Iranduba was slow and increasingly crowded. The crossing also cost up to $30 a passenger. So in 2010, Brazil built a two-mile-long cable-stayed bridge connecting the two cities. This bridge doesn't technically cross the main course of the Amazon; it crosses the Rio Negro, the Amazon's largest tributary. But it's the river system's very first bridge, and citizens rejoiced. Environmentalists, however, aren't fans of Manaus's new bridge and highway projects. In the past, road-building in the Amazon has been the first step to development and deforestation.

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