Ultramarathons, or ultras, are any running races beyond the standard marathon distance of 26.2 miles (or 42.2k). The most common ultra distances are 50k, 100k, 50 miles, or 100 miles, but each event is unique in terms of distance and terrain. The International Association of Athletics Federation recognises world records at ultramarathon events that have a distance of 100km. There are even 24hr ultramarathons, where the distance is unlimited, or multiday ultras, where the distance can extend beyond 1000km. There are a couple of city-based ultramarathons, but most ultras make the most of the USA's stunning scenery and landscapes. A lot of these events go off-road and require trail-running over difficult terrain with steep climbs or along rugged coastlines. There are over 1,500 ultramarathon events in the USA. One of the most famous and most difficult events is the Badwater Ultramarathon, which is 135 miles long through the searing heat and arduous terrain of Death Valley.