As British astronaut Tim Peake prepares to try to run the London Marathon virtually, on a treadmill in space on Sunday, these six races show that athletes with their feet on the ground have the same hunger for running in unusual environments:
Everest Marathon, Nepal
The world's highest marathon is definitely one of the most challenging. Competitors must hike for 15 days before they can even start, eventually reaching a height of over 5,000m before they set off downhill. Runners start from Gorak Shep, a frozen lakebed covered with sand, and finish at Namchee Bazaar, a village which sits at the dizzying height of 3,446m.
Great Wall of China Marathon, China
As if a marathon run wasn't hard enough, throw in 5,164 steps in with it, too. The run along the Great Wall of China undoubtedly supplies stunning views, but is definitely one of the most gruelling ways to cover the 26.2m distance in the world.
San Quentin Prison Marathon, USA
While many marathons are celebrations that take over entire cities, this particular outing is restricted to the inmates of California's oldest prison -- which also happens to be home to the greatest number of American prisoners on death row. Runners here repeatedly trudge around a four-acre box inside the prison grounds, so it's as much a mental challenge as a physical one.
Polar Circle Marathon, Greenland
What makes this marathon the most intriguing is the biggest risk involved: frostbite. It takes place in temperatures that reach as low as -15 degrees celsius and is surrounded by glacier tongues.There is also the serious threat off falling into a crevasse. Oh, and wild musk oxen and arctic foxes roam the track too.
Safaricom Marathon, Kenya
The fastest time this run has been completed in is 2 hours 18 minutes, 9 seconds, which is no surprise given that competitors are running through a game reserve full of hungry, wild animals -- including rhinos and buffalo. The course consists of two laps of a 13.1-mile route at an average altitude of 1,676m.
Volcano Marathon, Chile
Competitors assemble in the Atacama Desert -- the driest desert in the world -- before starting their run near Lascar, one of the most active volcanoes in Northern Chile. And if that wasn't enough, it is the highest desert marathon in the world, starting at 4,475m.