Along with the wildebeest, there are hundreds of thousands of zebra, nearly half a million gazelles, all of them crossing the territory of predators including lion, hyena, and cheetah. Nearby, the 60 Minutes spotted one cheetah with her three newborn cubs.
And the biggest predators of them all are crocodiles that patrol the Mara River, which cuts right through the migration route.
The river crossing is easily the most dramatic point in the entire year-long migration. There comes a time that the wildebeest and their calves have to cross the Mara River. You can't believe how big these crocodiles are - one of them is at least 15 feet long.
But the wildebeest have to cross in order to feed, and the crocodiles know that.
A wildebeest may go through 10 migrations in its lifetime. And to see them hesitate at the bank, it's as though many of them knew what was coming.
Pelley and the 60 Minutes team watched as first, two wildebeest scrambled across, making it across to the other riverbank.
Then the next group took the plunge, this time right into the waiting crocs. A large crocodile struck at lightning speed and had the wildebeest's horns between its jaws. And within moments, a total of five crocodiles were ferociously attacking the wildebeest all at once.
Now it was a struggle to find enough water to pull the wildebeest down to drown. In the few days that it takes the herds to cross the river, the crocs will bring down enough food to last for months.
Once the wildebeest see where the crocs are, the herd runs upstream and surges across by the hundreds. No one can say how long this migration has thrived, but on the Mara River we began to see evidence that its future is not a sure thing.
Usually the wildebeest swim across, but now the river is very low.
"Could what has happened to other migrations in the world happen here?" Pelley asked Robin Reid.
"Of course. Of course. Absolutely," Reid replied. "The thing I'm most worried about for the future is the Mara River and the amount of water in it. It's just the, you know, kinda the main artery of the ecosystem and it's very important."