Will a runner ever break the 2-hour barrier in the marathon? If you watched the finish of the 2017 Berlin Marathon on Sept. 24, you watched Eliud Kipchoge masterfully grind out a victory over Guye Adola in 2:03:32. It was an intriguing duel that illustrated how challenging it is to win a marathon nowadays but also how relatively far away we might still be to witnessing a runner break the 2-hour barrier in a race.

Kipchoge, along with Zersenay Tadese and Lelisa Desisa, took part in the Nike-sponsored “Breaking 2” project last spring, in which a time trial was set up at a Formula 1 track in Monza, Italy, with the intent of producing the world’s first sub-2-hour marathon. Backed by an elite group of physiologists and outfitted with custom shoes from Nike, those elite runners were given the chance to run for the once-unthinkable time amid near-perfect race conditions. In the end, Kipchoge was the one who came the closest, finishing in 2:00:25—slower than the goal but faster than anyone has run 26.2 miles ever before.

National Geographic recently released its “Breaking2” documentary about the time trial and has made the 55-minute episode available online.