At times I&rsquom convinced he&rsquos a non-human swimming robot,&rdquo river guide Matthew Mohlke says of Martin Strel, and you can see why. Strel, the Slovenian endurance swimmer, is a fearless conqueror of great rivers, the holder of the Guinness records for swimming the Yangtze, the Mississippi, the Danube and finally, in 2007, the Amazon. This is an account of the last of those marathons, a potentially deadly 3,274-mile journey in 66 days.