Serengeti National Park, in northern Tanzania, was established in 1952. It is home to the greatest wildlife spectacle on earth - the great migration of wildebeest and zebra. Seeking new pasture, the herds move north from their breeding grounds in the grassy southern plains. Many cross the marshy western corridor’s crocodile-infested Grumeti River. It is the migration for which Serengeti is perhaps most famous. Over a million wildebeest and about 200,000 zebras flow south from the northern hills to the southern plains for the short rains every October and November, and then swirl west and north after the long rains in April, May and June. So strong is the ancient instinct to move that no drought, gorge or crocodile infested river can hold them back. The Serengeti ecosystem is one of the oldest on earth. The essential features of climate, vegetation and fauna have barely changed in the past million years.