Theodore Roosevelt was never a fan of idle vacations. Whether ranching in the Dakotas, cougar hunting in Arizona, or going on a yearlong safari in Africa, his travels had always involved hardship and risk—two of the key components of what he once famously termed the “strenuous life.” Still, none of Roosevelt’s previous adventures could compare to the one he attempted in 1913. Despite having little experience in the jungle, the burly 55-year-old journeyed to Brazil and set out on a trip down an uncharted tributary of the Amazon: the mysterious Rio da Dúvida, or River of Doubt.

Roosevelt described the Amazon adventure as his “last chance to be a boy,” but it was also something of a consolation prize. He had hoped to begin serving a third term as president in 1913, but despite a strong showing in the 1912 election, he and his upstart Progressive Party had lost out to Democrat Woodrow Wilson. After moping around his New York home for a few months, Roosevelt received a letter from Argentina inviting him to conduct a series of lectures in South America. Not only did he accept, he decided to supplement the speaking tour with an extended river cruise down two tributaries of the Amazon. Before setting sail for the continent that October, he contacted the American Museum of Natural History, recruited a pair of naturalists and made plans to collect animal specimens during the expedition.

Roosevelt had envisioned a journey that was part holiday and part scientific endeavor, but upon arriving in South America, he decided to tackle something more stimulating. After consulting with his guide, the veteran Brazilian explorer Colonel Candido Rondon, he dropped his original itinerary and set his sights on traversing the River of Doubt, a wild and winding waterway that had yet to be charted by Europeans. The head of the American Museum of Natural History tried to warn him of the risks, but Roosevelt brushed off his concerns. “If it is necessary for me to leave my bones in South America,” he wrote, “I am quite ready to do so.”

In late-1913, after Roosevelt had completed his lecture tour, the “Roosevelt-Rondon” expedition got underway. Along with a small army of porters, explorers and scientists, the team also included Roosevelt’s 23-year-old son, Kermit, who had been living in Brazil. The adventurers began by traveling via steamboat to the remote town of Tapirapoan. From there, they embarked on a two-month overland trek toward the River of Doubt.

Though still carrying a bullet in his chest from a failed assassination attempt that occurred during his 1912 campaign, Roosevelt immediately impressed his companions with his seemingly boundless stamina. On the whole, however, the expedition did not get off to a promising start. Several men were struck down by tropical illness while crossing the rugged Brazilian highlands, and over half the group’s pack animals died from exhaustion. By the time they finally reached the River of Doubt in February 1914, a lack of supplies had forced Roosevelt and Rondon to downsize their team. In the end, the 22-man party that set off on the river included just three Americans—Roosevelt, Kermit and the naturalist George Cherrie.

If the journey to the River of Doubt had been trying, conditions only grew more extreme once explorers were on the water. As they floated down the river in dugout canoes, the men were at risk of attack by everything from alligators and piranhas to hostile indigenous people. Whenever they stopped to camp on its banks, they were overwhelmed by what Roosevelt called the “torment and menace” of mosquitos and stinging flies. Just a few days into the expedition, the former president had another run-in with the local wildlife when he was nearly bitten by a venomous coral snake. The creature snapped at his leg, but only managed to sink its teeth into his thick leather boot.

With each bend in the river, the expedition entered new and unmapped territory. “It was interesting work, for no civilized man, no white man, had ever gone down or up this river or seen the country through which we were passing,” Roosevelt later wrote. “The lofty and matted forest rose like a green wall on either hand.” The journey began on calm waters, but by early March the explorers had encountered the first of what would eventually be dozens of miles of tortuous rapids. At each cataract, the men were forced to either shoot the whitewater in their canoes or carry the boats on their backs through the wilderness. Their progress slowed to a plodding seven miles per day, and they had to repeatedly stop and build new canoes after several were destroyed during the crossings. On March 15, Kermit’s canoe was sucked into a whirlpool and sent tumbling over a waterfall. He and a companion managed to swim to shore, but a third man, a Brazilian named Simplicio, drowned in the rushing rapids.

The expedition’s troubles only mounted over the next several weeks. The explorers knew that a band of Indians was stalking them—Rondon had found his dog shot through with arrows—and they were constantly on edge about an ambush. The natives ultimately let the men pass unharmed, but the team was still plagued by malaria, dysentery and a lack of supplies. Even the indomitable Roosevelt began to suffer after he fell ill with fever and then sliced his leg open on a rock. Morale reached its lowest point in early April, when a porter named Julio shot and killed another Brazilian who had caught him stealing food. After failing to capture the murderer, the exhausted expedition simply abandoned him in the jungle.

The 19 remaining explorers continued downriver, but their scientific expedition had turned into a fight for survival. Their clothes were reduced to rags, and they headed off starvation only by catching fish and scrounging for hearts of palm. Roosevelt, once among the team’s strongest members, became delirious from fever and infection. He repeatedly demanded to be left alone in the jungle to die, but Kermit refused to leave him behind. “There were a good many days, a good many mornings when I looked at Colonel Roosevelt and said to myself, he won’t be with us tonight,” naturalist George Cherrie later remembered. “And I would say the same in the evening, he can’t possibly live until morning.”

Roosevelt eventually lost a quarter of his body weight, but he stubbornly held on and even endured emergency leg surgery on the riverbank. As the former president languished in his canoe, Rondon led the explorers into waters closer to civilization. With the aid of local “seringueiros”—Brazilian pioneers who lived in the jungle and harvested rubber—the men acquired new canoes and traversed the last few sections of rapids.

Finally, on April 26, the team sighted a relief party that Rondon had previously ordered to meet them at the confluence of the River of Doubt and the Aripuanã River. After two months and hundreds of miles, they had reached the finish line. Though still sick, Roosevelt beamed with pride. In typically stoic fashion, he dashed off a telegram to the Brazilian government in which called the nightmarish expedition “a hard and somewhat dangerous, but very successful trip.”

Roosevelt received medical attention once the group reached civilization, and by the time he returned to New York in May 1914, he had grown strong enough to walk down his ship’s gangplank and greet a crowd of admirers. A few critics tried to dispute his claim that the expedition had “put upon the map a river nearly 1,500 kilometers in length,” but he later won over most of the skeptics during an extended lecture tour. In 1926, meanwhile, another group of explorers repeated the river journey and confirmed nearly all the Roosevelt-Rondon expedition’s geographical findings. By then, the Brazilians had given the River of Doubt a new name: the Roosevelt River.

While Roosevelt would remember his time in the Amazon as one of his greatest adventures, it was also his last. His stint in the jungle had taken its toll, and for the rest of his days he was plagued by a collection of ailments he called his “old Brazilian trouble.” The venerable “Bull Moose” stayed active and even attempted to volunteer for World War I, but he finally died in his sleep in 1919 at the age of 60. “Death had to take him sleeping,” Vice President Thomas Marshall said at the time, “for if Roosevelt had been awake, there would have been a fight.”