The migration follows the rain. When the grass in Tanzania's Serengeti grows tall after the wet season, the herds eat their way north. When that grass is gone, they keep walking - across rivers - to find new grass in the Maasai Mara in Kenya.
The most famous moment is the river crossing. The wildebeest have to swim across the Mara River. Thousands cross at once, jumping in together because it is safer in a giant group than alone.
The whole loop is about 800 kilometres long - roughly the distance from London to Edinburgh and back. The animals do it every single year, generation after generation, without maps.
The migration changes the landscape wherever it goes. The grass gets shorter, the soil gets richer, and millions of birds and insects follow along to find food. When the herds leave, the land starts growing back ready for the next year.