It's a crazy plan: to ride down the world's longest descent from the roof of the Andes at 6890m to the Pacific coast of Chile - 300Km in all. But to even get to the start line, James Lowsley-Williams and Mark Beaumont need to scale the world's highest active volcano, Ojos del Salado with their bikes strapped to their backs. To make the start line they must acclimatise to the crushing altitude and the barren landscape before their attempt begins. Mark and James have nailed several epic rides in the past, but this latest attempt moves them into a different league, with extreme altitude and barren inhospitable landscapes challenging them before they can even begin.