Christmas Island red crabs
Endemic to the remote Christmas Island in the Indian Ocean, the red crabs embark on a seasonal migration that is truly one of nature's grandest spectacles. Every year during breeding season they transform the island into a vast moving red carpet as they scurry to the ocean to lay their eggs.
Anywhere from 40 million to 120 million crabs are believed to blanket the island, and during peak migration roads must often be closed. Here is a video from Parks Australia documenting the annual event that has to be seen to be believed.