Mountain bike racing (shortened MTB or ATB racing) is the competitive cycle sport discipline of mountain biking held on off-road terrain. The Union Cycliste Internationale (UCI) recognised the discipline relatively late in 1990, when it sanctioned the world championships in Durango, Colorado. The first UCI Mountain Bike World Cup series took place in 1988. Its nine-race circuit covered two continents—Europe and North America—and was sponsored by Grundig. Cross-country racing was the only World Cup sport at this time. In 1993, a six-event downhill World Cup was introduced. In 1996, cross-country mountain biking events were added to the Olympic Games. In 2006, cross-country mountain biking events became part of the World Deaf Cycling Championships for the first time in San Francisco, USA. In the United States, there are three USA Cycling Mountain Bike National Calendars: Endurance, Gravity and Ultra-Endurance. USA Cycling runs the USA Cycling Mountain Bike National Championships. There are mountain bike racing types that are not recognized by the UCI, such as mountain bike orienteering that is governed by the IOF. History Mountain bike racing is as old as the first appearance of the mountain bike itself, when the originators, calling themselves klunkers, descended with their heavily modified beach cruiser bikes as predecessors of modern mountain bikes from numerous mountains in California against a rudimentary time measurement. The famous Repack racing down-hill against the time near Fairfax, Marin County, CA took place in 1976. The oldest organised cross-country (XC) racing-like event, however, is believed to take place from Crested Butte to Aspen in Colorado in 1978. It took 10 years before the UCI recognised racing on mountain bikes as a regular cycling discipline in 1990. In the meantime, the phenomenon of mountain bike racing spread across the U.S.A. under the umbrella of NORBA. The most plausible, although still speculative reason, was an early resemblance of the racing courses for mountain bikes with the cyclo-cross discipline, with a major difference: the mountain bike racing track was significantly longer (a cyclo-cross course has 2.5–3.5 km versus 4+ km for a XC track) and free of artificial obstacles. With the rapid advancement of mountain bike gear, namely stronger brakes and suspension, the mountain bike racing could take place on even more technical tracks making it more dissimilar to the cyclo-cross discipline. Mountain bike racing became an Olympic discipline in 1996 and a cross-country type mountain biking race has been held ever since