Over a million animals follow the same thousand-mile loop every year

Each year, roughly 1.5 million wildebeest circle through the Serengeti and Masai Mara following the rains and fresh grass, crossing crocodile-filled rivers in the process. It's the largest overland mammal migration left on Earth, and it's so consistent that the herds' movements can be predicted almost to the week, year after year.

— A.R.E. Sinclair, Craig Packer, Simon A.R. Mduma, and John M. Fryxell (eds.), Serengeti III: Human Impacts on Ecosystem Dynamics — University of Chicago Press, 2008