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The annual migration of millions of wildebeest across the plains of Africa is one of Nature's most spectacular events. What drives this migration is rain and the search for food. Every year, wildebeest, zebra and antelope migrate clockwise around the Serengeti / Masai Mara ecosystem. In January the herds can be found in Tanzania's Serengeti heading south into southern Serengeti where they calve. By July the herds have reached western Serengeti and the Grumeti and Mara rivers. The river crossings are their biggest challenge. Nile crocodiles which can be as much as 5 metres in length long crocodiles lie in wait. On the river banks, the wildebeest jostle against one another. Animals at the front slither about on the mud before plunging into the river and a stampede follows. The crocodiles seize their chance, lunging out of the water at the terrified baying herd. Hours become days and the river becomes a blood bath. Eventually the crocodiles are satiated. Vultures arrive to pick over the dead and dying. It's a scene of carnage. The sounds are chilling. In time the crocodiles drift away, the waters become restful, and tranquillity is restored. Hippos bathe and the wildebeest continue their long migration into the Masai Mara reserve. Producer Sarah Blunt.