The Elfstedentocht is a long-distance tour skating event on natural ice, almost 200 kilometers long, which is held both as a speed skating competition (with 300 contestants) and a leisure tour (with 16,000 skaters). The Elfstedentocht is the biggest ice-skating tour in the world.

The tour is held in the province of Friesland in the north of the Netherlands, leading past all eleven historical cities of the province. The tour is held at most once a year, only when the natural ice along the entire course is at least 15 centimeters thick; sometimes on consecutive years, other times with gaps that may exceed 20 years. When the ice is suitable, the tour is announced and starts within 48 hours.

The tour, almost 200 km in length, follows a closed or circular route along frozen canals, rivers and lakes visiting the eleven historic Frisian cities: Leeuwarden, Sneek, IJlst, Sloten, Stavoren, Hindeloopen, Workum, Bolsward, Harlingen, Franeker, Dokkum, then returning to Leeuwarden. The tour is held only if the ice is, and remains, at least 15 centimeters thick along the entire course as about 15,000 amateur skaters take part, putting high requirements on the quality of the ice. The last tours were held in 1985, 1986, and 1997. All skaters must be members of the Association of the Eleven Frisian Towns. A starting permit and bib are required (€100 in 2017). Skaters must collect a stamp in each city, and at three secret checkpoints, and must finish the course before midnight.

The finishing point of the Elfstedentocht is a canal near Leeuwarden, called the “Bonkevaart”, close to the landmark windmill, De Bullemolen, Lekkum.

The Elfstedentocht has been declared to be in danger of “extinction” due to climate change. In the past 50 years, the Elfstedentocht has taken place only three times, most recently in 1997.

According to wikipedia