It 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.
South Africa is also home of the world's largest ultra-marathon, the Comrades Marathon, as well as the world's largest (the Cape Argus Cycle Race in Cape Town) and second-largest (the 94.7 cycle challenge in Johannesburg) timed cycle races.