Every year, like clockwork, millions of wildebeest risk it all on an epic round-trip journey across the African Serengeti.

Known as the Great Migration, it’s actually the largest terrestrial migration in the world. During the journey, the animals travel more than 1,000 kilometers from the southern Serengeti in Tanzania all the way up to the Masai Mara Park in Kenya and back.

All told, the number of animals on the move include more than 1.5 million wildebeest, 200,000+ zebra and thousands of other grazing animals travelling on the periphery.

But as the video above shows, the migration can be a perilous endeavor.

For one, the wildebeest calving season begins right before they set off on the year-long trek. From January to February, hundreds of thousands of pregnant females give birth en masse over a 2-3 week period and their young are a prime target for predators. Cheetahs, leopards and lions often track the herds along their path, waiting to pick off the feeble calves, which easily stand out among the adults.

Then, there are the river crossings. The Grumeti River in Tanzania and the Mara River in Kenya are infamous for the ferocious crocodiles lurking below the surface. Hordes of wildebeest and zebras wade through the water together, but not all will make it to the other side. Inevitably, hungry crocodiles will attempt to pull some of the animals under.

(See: Wildebeest Escapes Jaws of Crocodile With Unexpected Help From Zebras)

Even when they do escape, injured animals might not live to see the end of the trip.

In fact, by the time they complete their journey, around 250,000 migrating animals will have died either at the hands of these carnivores or from fatigue and hunger.