20 Animal Journeys That Still Defy Explanation

When the first wet-season rains arrive on Christmas Island, the forest floor begins to change. Millions of red crabs move from inland forests toward the sea, turning roads, paths, and shaded ground into a slow red procession. Their timing is not random. Rainfall helps trigger the movement, while lunar cycles and tides help determine when spawning should happen. People on the island even build crab bridges and close roads to help them pass safely.