Photographer David Clifford has captured the world's most dogged runners doing what they love in the most remote and unforgiving environments.
1. Giant's Causeway
Ta-da! Declared a world heritage site by Unesco in 1986, Giant's Causeway is Northern Ireland's most popular attraction. This natural wonder was formed during the Palaeogene period, 50 to 60 million years ago, and is the result of a huge mass of molten lava being spewed out onto the surface of the Earth to form up to an estimated 37,000 black basalt columns.
While it's a relatively simple process, when you take a look at the geometric shapes formed, it's hard to believe that the production was entirely natural. And this protected landmark, which is run by the National Trust, was voted your number one in Britain's greatest.