Over the last few days, you’ve enjoyed beautiful Christmas Island. You hiked through the dense rainforest to the Dales, seven watercourses with streams, springs, and waterfalls that lead toward the sea. You found Hosnies Spring deep within the mangroves. You sighted two types of endangered birds: the Christmas Island frigatebird and the Abbott’s booby. You jumped into the clear water from high limestone cliffs, explored underground caves, and splashed in rock pools. It’s been quiet and peaceful out in the middle of the Indian Ocean.

That’s about to change. It’s also been raining the past couple of days. Sometimes it was a light drizzle, while other times, it was more of a downpour. You were having too much fun to really notice, though. But others were paying close attention to the beginning of the wet season. And enticed by the showers, they’re ready to come out of the rainforest.

Christmas Island’s annual red crab migration is about to begin. Every year, 100 million—yes, 100 million—red crabs move from the forest to the shore to breed. There are so many of the five-inch-wide crabs that a red mass can be seen from the sky. Up close, there’s a constant clattering sound as their claws move over rocks, branches, roads, and eventually sand. The males go first, to start burrowing in the sand. The females follow to lay their eggs, which will later be released into the ocean.

It’s a good thing you explored so much of the Australian territory when you first arrived. Like everyone else on the island, you can’t take your eyes off the red crabs right now. Not that you could avoid so many of them anyway. Like the trip, it’s a once-in-a-lifetime experience.