After six months of scientifically advanced training, three of the world's most elite distance runners set out to break the two-hour marathon barrier. These pioneers go on a global trek to defy the unthinkable and break the two-hour feat, from testing in wind tunnels and running labs in the United States, to balancing training with their day-to- day lives in eastern Africa, to the final heart-pounding race in Italy.

• National Geographic
• Eliud Kipchoge
• Zersenay Tadese
• Lelisa Desisa