Every year, some time around July, 1.5 million wildebeest gather on the southern banks of the Grumeti river in Tanzania, eying their chances against the formidable crocodiles and its coursing waters as they leave behind the arid grasslands of the Serengeti for the wetter northern woodlands of the Masai Mara in Kenya.

This is the Great Migration, and it's the largest mass-movement of land mammals on the planet.

The females would have calved between January and March. The timing would have been dependent on the rutting season, sometime in the previous May or June. But as the dry season comes, the plains dry out, and the young reach the stage when they are capable of embarking on a 1,500 kilometre journey, they begin to move.

They head north, following the rains.

Once they have made their way across the Grumeti and cantered north, the herd will hit the Mara river, another crocodile-ridden battle ground. Not all the animals will make it. They are a feast for crocodiles, yes, but they also fall victim to broken legs as they jump from the banks and into the river, they are swept downstream by the current, or they're prey for lions and hyenas.

Those who safely navigate it will be able to feast on the lush pastures of the Serengeti until October or November.

By then, the rains will have come to the south and the plains will have been rejuvenated, made verdant again. Once again, the herd masses, contemplates the river, and eventually one animal will make the plunge and lead them back on their journey to their breeding grounds, so that the cycle can begin once again.

It's chaotic, it's noisy, and it's an incredible spectacle.

(You can also see the most awesome video of the Great Migration, as the wildebeest return south, and face the Mara river a second time around, made by Will and Matt Burrard-Lucas.)