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After Theodore Roosevelt lost the American presidential election of 1912, he decided to explore an unknown tributary of the Amazon called the River of Doubt.

It was a trip that nearly killed both Roosevelt and his son, Kermit.

Witness brings together first-hand accounts of the trip, with the help of Candice Millard, author of River of Doubt - Roosevelt's Darkest Journey.

PHOTO: Roosevelt on the campaign trail in 1912 (Getty Images).