British adventurer Sir Ranulph Fiennes is running the Marathon des Sables, which some call the toughest race in the world.
He will have to travel more than 150 miles across the Sahara desert, in temperatures of 50C and carry all his supplies while he runs too. But is it the most difficult race out there?
Here's Newsround's guide to some of the toughest challenges in the world...
The Arctic Circle Ski Race
The ultimate endurance race involves skiing 100 miles across the Arctic in temperatures as low as minus 50C. It lasts three days and the nights are spent trying to keep warm in tents. Skiiers are accompanied by supporters travelling on sleds pulled by dogs.
The newest event on our list claims to be the race to end all races. The Jungle Ultra requires you to travel through the Amazon Rainforest in Peru. Carrying your hammock (to sleep on each night), food supplies, sleeping bag and the rest of your kit. Runners go through jungle trails and wade through 70 river crossings, spending the evenings in the villages of local tribes.
Yukon River Quest
The world's longest annual canoe and kayak marathon is 444 miles through the Yukon River, in North America. Participants race around-the-clock for five days in freezing waters. The event takes place in the summer so there is 24 hours of daylight, which is why some people call it the Race of the Midnight Sun.
Dakar Rally, South America
This race across South America is open to professional and amateur drivers who can choose to travel by car, truck, quad bike or motorcycle. It is a coast to coast journey through Argentina, Chile and Peru across the Andes Mountains, the Atacama Desert and between the shores of both the Atlantic and Pacific Ocean.