The Marathon des Sables (MdS, Marathon of the Sands, or Sahara Marathon) is a six-day, 251 km (156 mile) ultramarathon, which is the equivalent of six regular marathons. The longest single stage (2009) was 91 km (57 miles) long. This multiday race is held every year in southern Morocco, in the Sahara desert. It's considered the toughest foot race on Earth. Competitors must carry all personal belongings and food for the entire event in their backpack and it can get up to 120 degrees. The terrain can be pretty tough too. Water (which is rationed) and tents are supplied by the organizers. You can read more about it here, and watch a video here too.