Photographer David Clifford has captured the world's most dogged runners doing what they love in the most remote and unforgiving environments.
Yarlung Tsangpo Canyon, Tibet and Fish River Canyon, Namibia
Many people think the Colorado Grand Canyon is the largest in the world. However, the American Geography Committee announced in 1994 that it was in fact the Great Canyon of Yarlung Tsangpo (Brahmaputra) River in the Himalayas (Tibet) that claimed this title. The giant fissure descends to an astounding 5,382 metres at its deepest point and its length stretches for more than 496km. Another little-known claimant for the world’s biggest gorge is Fish River Canyon, a spectacular site in Namibia.