Photographer David Clifford has captured the world's most dogged runners doing what they love in the most remote and unforgiving environments.
Inspire your inner travel photographer...
All of these beautiful images are in the running for the 2012 National Geographic Photo Contest, and if they don't awaken your inner photographer, we don't know what will. If you think you can do better, enter the contest here, but first click through our gallery to see what you're up against.
Kirk Nielsen, who took this stunning photograph, says: "As I am sitting on top of a suspended submersible cage, 30 feet below the surface, a 15-foot Great White Shark swims overhead just starting to block out the sun. I took the shot right after I had released a breath though my respirator."
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Worldwide,it would be possible to amass a vast collection of excellent pictures of a similar kind to those portrayed here.With the www,time and cost would be hugely reduced,but a number of volumes could be created by those grouped with the common interest.
Whereas,nature,such as lightning and glaciers could accomodate one type of volume and wildlife,such as penguins and turtles,another type of volume,city pictures created by humankind is really a different realm of its own.