The Midmar Mile is a swimming race held annually in February at the Midmar Dam north of Pietermaritzburg, South Africa. Each year, it draws thousands of competitors, from serious international athletes and Olympic medallists to purely recreational swimmers.

In 2009 the event, in which 13 755 compentitors finished the race, was recognised by the Guinness Book of World Records as the world's largest open water swimming event.[

The event gains its name from the location (Midmar Dam) and the distance (roughly one mile). A unique feature of the race is that while the distance covered is always a mile, depending on rainfall and the water levels in the dam, the distance swum varies from year to year. In years with poor rainfall, competitors are subject to the infamous Midmar sprint start: a bedlam of bodies sprinting across the muddy lake-shore and through the shallows until the water is deep enough to swim.

In order to handle the vast number of competitors, the swimmers swim out in several groups at two-minute intervals in 8 batches over two days; the group division is based on a qualifying time in a previous event, with the fastest group leaving first.

Source: Wikipedia