The Ultra Trail Du Mont Blanc sees ten thousand (!) runners take on a 166km race with an elevation of 9,600m every year.

Held in the French Alps the race takes between 20 hours for the fastest runners, to 45 hours for the slower end.

The race now has prize money but is wholly uncompetitive, with runners travelling in groups of two or three for safety.

This route is a bit of a life goal for many trail runners, with past participants talking of the absolute beauty of the race.