Every year, two million wildebeest, zebra, and gazelle move in a circular migration between Tanzania's Serengeti and Kenya's Maasai Mara, following the rains and the grass. They cross the Mara River in July and August, a chaotic spectacle of hooves and water and crocodiles. The migration is the reason the Mara exists as a reserve, the foundation of Kenya's tourism industry, and one of the last great wildlife phenomena on earth. It is not a parade. It is survival, repeated annually, indifferent to borders.