Triathlons are multisport events, which consists of sequential stages of swimming, cycling and running.There are five official Triathlon distances: Sprint Distance (750m swim, 20km bike, 5km bike); Olympic (1.5km swim, 40km bike, 10km swim); ITU Long (3km swim, 80km bike, 20km run); Half/Middle-Distance/70.3/Half-Ironman (1.9km swim, 90km bike, 21.1km run); Full/Ironman (3.8km swim, 180km bike, 42.2km run). There are other unofficial distances, such as super-sprint triathlons or ultra-triathlons. Multisport events have been growing in popularity in the USA over recent years, particularly triathlons. USA triathlon, the national governing body, now has over 500,000 members, with over 4,000 races each year. Many of these triathlon events are city-based, with the swim section in pools or manmade lakes, and the cycle and run elements through city streets and past famous landmarks. Other triathlons require open-water swimming in the sea, and can also require mountian biking and trail running as they head off-road onto challenging terrains. There are multisport variations of triathlons such as duathlon, aquathlon, and aquabike, which require two of the disciplines but not all three.