An ultra-marathon is a footrace longer than a marathon (26.2 miles or 42km). However ultra marathons can also be arranged over time (for example 24 hours or multi-day), as well as distance. For elite competition, the IAAF has races over 100km.
Ultra-marathons have enjoyed significant growth over the past few years. In 2017 in the USA over 100,000 people completed an ultra-marathon and many events sell out as soon as they go on sale.
Ultra-marathons are not just a recent phenomenon. The history of the sport can trace its way back to the original London to Brighton footrace in 1837. Two professional runners; John Townsend and Jack Berry. The 45 year old Townsend was victorious in 8 hours 37 minutes.
One of the most famous ultra-marathons in the world, the Comrades started in 1921. This event from Pietermaritzburg to Durban in South Africa. The route reverses each year with "up" and "down" years depending on the direction. The first event included 34 participants. The event now attracts 18,000 participants each year.