One of the greatest feats of human endeavour is underway in Yosemite. Two of the world's best climbers are navigating the marble-smooth Dawn Wall, the last unconquered side of El Capitan, the 900m rock.

John Long, climbing author and the team's media manager, told Today: "They're about halfway up. These guys have been working on this project for eight years. They've memorised every hold from the bottom to the top."

"If all things go well ...the best guestimate is they'll be done next Tuesday."

First broadcast Today programme 7 January 2015.