When Teddy Roosevelt received a letter from Argentina inviting him to visit and give some lectures, he was at a low point, having recently lost the Presidency, and not a little worried about money, having invested most of his inheritance in politics. A trek to South America would allow him to restock the coffers, revitalize his dented ego and visit his son Kermit, who was working in Brazil.

Just sailing out of New York made Roosevelt “feel like Christian in Pilgrim’s Progress when the bundle fell from his back,” as his wife Edith described it. Roosevelt was determined to be no ordinary tourist. An invalid as a child, he had built up his strength and health by dogged efforts of will, and let nothing deter him from bold acts of risk-taking. So when Brazil’s Minister of Foreign Affairs proposed a journey down the Rio da Dúvida, the River of Doubt, Teddy was just audacious enough to jump at the chance.

The river was all but unknown and attempts to chart it had resulted in the death of at least one noted explorer. Yet Roosevelt regarded the proposed adventure as just “slightly hazardous.” He had always wanted to explore something, but most places on earth were already mapped. The project was his last great achievement. He died a few years later, possibly from a flare-up of problems that started with a septic wound sustained on the South American journey. He was eulogized as a quintessential American hero.

Candice Millard, former writer and editor for The National Geographic, has drawn together Roosevelt’s own accounts, Kermit’s diaries and numerous lectures to create a nonfiction account that begs to become a movie. As she states it, “the moment the men of the expedition pounded their camp markers into the riverbank, the Amazon began to dismantle them.” Their mark was made in the civilized world, while the jungle obliterated all trace of their brave passage.

The details of the expedition are fascinating, from the men’s choice of reading matter for the journey (The Oxford Book of French Verse was a big hit) to the ongoing strife with native tribes, the loss of life and equipment, and the catalogue of Roosevelt’s personal sorrows serve as a backdrop to the immediacy of death and danger in the tropics.

An unforgettable moment comes when Roosevelt must submit to an operation on his leg under the crudest of conditions. “As the doctor worked to insert a drainage tube, swatting away the legions of piums and borrachudo flies that had been attracted to the stench, Roosevelt never cried out in pain or uttered a word of complaint.” Yet for him, the great accomplishment was putting the River of Doubt on the map.