Jorgesson's hands grip the razor edge on pitch 15. (Tom Evans)

Tommy Caldwell and Kevin Jorgeson trained for years with one goal in mind — to climb El Capitan's Dawn Wall in Yosemite National Park, known as one of the most challenging climbs on earth. The nearly-smooth rock presents a 90-degree climb for more than a half mile.

To make the climb even more challenging, the two are ascending the wall as a free-climb, without the aid of ropes, except to catch their falls. This type of climb has never been done before on the wall, and , if they reach the top, photographer Tom Evans told The New York Times.

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The climbers have been at it since Dec. 27. At night and during breaks, Jorgeson and Caldwell set up base camp, which consists of two tents seemingly precariously hung from carabiners and ropes.

What makes the climb so hard is not only the steepness of the wall, but the smoothness. Hand holds are tiny, some as small as matchsticks, according to The New York Times. The everyday climbing can cause fingertips to rub raw. During warmer days, Caldwell and Jorgeson are forced to climb after sunset, as sweaty hands make the climb even more tough.

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This isn't Caldwell and Jorgeson's first attempt at the Dawn Wall. The two attempted the climb in 2010, but storms botched the ascent about a third of the way up. Jorgeson broke an ankle during a 2011 climb, and Caldwell proceeded, but encountered difficulties at the climb's forteenth pitch, the toughest pitch of the route. When training for this time around, the climbers decided to master each pitch individually before ascending the entire wall.

"This is my Moby-Dick, for sure," Caldwell told The New York Times. “For me, it’s just a fascination with the epic journey. I’ve always been a fan of stories of big journeys. And it’s a question of curiosity. I love to play with my physical and mental limits and see how far I can push them, and I just love to dream big. And this project fulfills all those things.”