There are two types of ultramarathon events: those that cover a specified distance, and events that take place during specified time (with the winner covering the most distance in that time). The most common distances are 50 and 100 miles, or 50 and 100 kilometers.
The "100 kilometers" is an official IAAF event as can be seen at http://www.iaaf.org/statistics/records/inout=o/discType=5/disc=100K/detail.html.
Other distances/times include double marathons, 24-hour races, and multiday races of 1000 miles or even longer. The format of these events and the courses vary, ranging from single or multiple loops (some as short as a 400-meter track), to point-to-point road or trail races, to cross-country rogaines. Many ultramarathons, especially trail challenges, have severe course obstacles, such as inclement weather, elevation change, or rugged terrain. Many of these races are run on dirt roads or mountain paths, though some are run on paved roads as well. Usually, there are aid stations every 5 to 15 km apart, where runners can replenish food and drink supplies or take a short break.
Timed events range from 6, 12, and 24 hours to 3 and 6 days (known as multi-day events). Timed events are generally run on a track or a short road course, often one mile or less.
The International Association of Ultra Runners (IAU) organises the World Championships for various ultramarathon distances, including 50K, 100K, 24 hours and ultra trail running. These events are sanctioned by the International Association of Athletics Federations (IAAF), the world governing body of track and field. Many countries around the world have their own ultrarunning organizations, often the national athletics federation of that country, or are sanctioned by such national athletics organizations. World records for distances, times and ages are tracked by the IAU.
Ultramarathons are run all over the world and over 70,000 people complete ultramarathons every year. A list of the most popular ultra marathons can be found under the subsection "Well-known ultramarathons" below.
Several ultra distance events are held in Africa. South Africa hosts the world's oldest and largest ultramarathon, the 89 km Comrades Marathon. Approximately 12,000 runners complete Comrades each year, with over 24,500 in 2000. It also hosts the 56-kilometer Two Oceans Marathon in Cape Town in the southern autumn which attracts approximately 7000 runners. Marathon des Sables is a 7 day stage race which covers 250 km through the Sahara desert in Morocco. The Sahara Race in Egypt, part of the 4 Deserts series, is held annually with about 150 competitors from 30 countries competing. There is also an ultramarathon of 250 km across the Namib desert.
Ultrarunning has become popular in Asia recently, and countries such as Taiwan, Japan, and Korea have hosted IAU World Championships in the last few years. Korea's first ultramarathon was held in 2000. India's first ultra marathon was held in 2007, in Bangalore. The Gobi March, first held in 2003, in northwest China was China's first ultramarathon. Singapore has a double marathons night race called Sundown Marathon  since 2008. In Nepal, the Kanchenjunga Ultra Marathon Trail Running Race, first held in 2009, started from Phunlin Bazaar (near Taplejung) at the Nepal and Sikkim border.
The first ultramarathon held in New Zealand was on a 100 km track. Australia and New Zealand are host to some 100 ultramarathons each year. One of the most famous Australian ultramarathons was the Westfield Ultra Marathon, an annual race between Sydney and Melbourne which was contested between 1983 and 1991. Greek runner Yiannis Kouros won the event five times during that period. Australia is also the home of one of the oldest six-day races in the world, the Cliff Young Australian 6-day race, held in Colac, Victoria. The race is held on a 400-meter circuit at the Memorial Square in the centre of Colac, and has seen many epic battles since its inception in 1984. The 20th Cliff Young Australian 6-day race was held between 20–26 November 2005. During that event Kouros beat his existing world record 6-day track mark and set a new mark of 1036.851 km. The Coast to Kosciuszko inaugrated in 2004, is a 246 kilometres (153 mi) marathon from the coast to the top of Mount Kosciuszko, Australia's highest mountain.
Ultrarunning is also popular in Europe, where over 200 ultramarathons are held each year. Among the biggest are:
The second oldest ultramarathon in the world, London to Brighton, was widely considered to be among the most prestigious titles until its retirement in 2005. The earliest written documentation of ultrarunners came from Icelandic sagas. The history of ultrarunners and walkers from the Victorian Era has also been documented. The IAU hosts annual European Championships for the 50k, 100k and 24 hours.
The European Ultramarathon Cup (ECU) is an annual series covering several of the biggest races in different European countries.
Due to logistics, environmental concerns and travel costs (also for organisers) there are only a handful of ultramarathons in Antarctica including: The Last Desert, a multi-stage footrace, and the Antarctic Ice Marathon - a marathon and 100-kilometer race.
There are several hundred ultramarathons held annually in North America. One of the most popular is the Western States Endurance Run, the world's oldest 100-mile trail run. The race began unofficially in 1974, when local horseman Gordy Ainsleigh's horse for the 100-mile Tevis Cup horse race came up lame. He decided to travel the course on foot, finishing in 23 hours and 47 minutes.
One of the first documented ultramarathons in North America was held in 1926, as part of the Central American Games. Tomas Zafiro and Leoncio San Miguel, both Tarahumara Indians, ran 100 km from Pachuca to Mexico City in 9 hours and 37 minutes. At the time, the Mexican government petitioned to include a 100 km race in the 1928 Summer Olympics in Amsterdam, however nothing came of these efforts.
In 1928, sports agent C. C. Pyle organized the first of two editions of the 3455-mile-long Bunion Derby (the first went along U.S. Route 66 from Los Angeles to Chicago before heading toward New York; the 1929 Derby reversed the route). Neither the race nor the accompanying vaudeville show was a financial success.
In April 2006, the American Ultrarunning Hall of Fame was established by the American Ultrarunning Association (AUA). Candidates for the Hall of Fame are chosen from the 'modern era' of American ultras, beginning with the New York Road Runners Club 30 Mile race held in 1958. The Inaugural inductees were Ted Corbitt, a former US Olympian, winner of the aforementioned race in 3:04:13, and co-founder of the Road Runners Club of America, and Sandra Kiddy, who kicked off her ultra career at age 42 with a world record at 50 kilometers, 3:36:56, and who went on to set a string of US and world ultra records.
Wickham Park Marathon, 50M, 100M, 200M
Extra long distance bicycle races are also referred to as "ultramarathons" or "ultracycling" events. In cycling, there is no specific distance that is directly analogous to runners' 26 miles (42 km), so the definition of cycling ultramarathon is not set in stone. Generally, all events that last 12 hours or longer or cover distances of over 200 miles (so-called "double century") can be considered ultramarathons. Some examples of ultramarathons include Race Across America and its qualifying races.
An ultramarathon is a running race that is longer than a 26.2 mile marathon race. The most common distances for ultramarathons are:
50 Kilometers (31 miles) 50 Miles (80.5 kilometers) 100 Kilometers (62 miles) 100 Miles (161 kilometers)
The most famous ultramarathon run in the United States is the Western States 100 Mile Race.