The River of Doubt—it is a black, uncharted tributary of the Amazon that snakes through one of the most treacherous jungles in the world. Indians armed with poison-tipped arrows haunt its shadows; piranhas glide through its waters; boulder-strewn rapids turn the river into a roiling cauldron. After his humiliating election defeat in 1912, Roosevelt set his sights on the most punishing physical challenge he could find, the first descent of an unmapped, rapids-choked tributary of the Amazon. Together with his son Kermit and Brazil’s most famous explorer, CÒandido Mariano da Silva Rondon, Roosevelt accomplished a feat so great that many at the time refused to believe it. In the process, he changed the map of the western hemisphere forever.