Fifty million crabs march to the sea on Christmas Island every year

Each October or November, monsoon rains trigger roughly 50 million red crabs to leave the rainforest and walk to the ocean in a single wave. Males dig burrows and mate first, then the whole population converges on the shoreline. Females time their spawn to the exact night of the last quarter moon, releasing eggs into the surf within minutes of each other.

— Wikipedia contributors, Christmas Island red crab — Wikipedia, citing Parks Australia field data