Dunes day is traditionally Day 3 of the Marathon des Sables. In the 2000 race, the dunes section was a 20 kilometre section of continuous dunes with no water replenishment. Competitors had to first travel around 13 kilometres to get to the dunes and a further 4 kilometres after completing the dunes section - a total of 37 kilometres on Day 3!

(This picture is shown with kind permission from Nick Dillon)