The Great Wildebeest Migration is an enormous year-round movement of animals in which close to 2.5 million herbivores munch their way along an 800-km route around the greater Serengeti area of north-west Tanzania, occasionally dipping into the Maasai Mara area of south-west Kenya. The main body of the migration is formed by around 1.7 million wildebeest and 500,000 zebras. This mass movement is driven by the continuous search for drinking water and quality grazing, which is in turn dictated by the seasonal pattern of rainfall and the underlying soil conditions. The migration is accessible for most of the year, although you do very much have to plan your safari to be in the right place at the right time. The three main highlights are the river crossings from June to October, when Sanctuary Olonana on the banks of the Mara River is perfectly sited, the calving season from December to March and the rut from March to May.