Millions of bright red crabs have begun their yearly mating migration from the forest to the sea on Australia’s Christmas Island. The scarlet crabs are marching from the jungle toward the Indian Ocean, swarming roadways, parks, and residential neighborhoods, so they can mate in or near burrows by the water. Many roads are closed and specially-made crab bridges have been erected to allow the crabs to more safely cross the larger thoroughfares.

The Washington Post reports that according to the Australian government, some 50 million red crabs live on Christmas Island and Cocos (Keeling) Islands — the only places in the world where they can be found. The phenomenon happens each year and is determined by the phase of the moon, according to Parks Australia, which calls the mass migration “one of the most incredible natural processes on Earth.” 

“The crabs are turning up everywhere, including at the door of an office block,” Parks Australia tweeted this week, adding that the yearly migration was “in full swing.”

The natural migration process is so impressive that world-famous naturalist Sir David Attenborough listed it as one of the top TV moments of his career, calling it “an astonishing, wonderful sight.”

“It is like a great scarlet curtain moving down the cliffs and rocks toward the sea,” he said, adding that the creatures treated him as though he was just another obstacle on their path toward the sea, climbing up his legs as they continued their journey.

“That’s how I discovered how difficult it is to deliver lines while several four-inch crabs, each armed with sharp claws, are advancing menacingly up your inner thigh,” he joked.

Christmas Island turns red as millions of crabs march to sea for annual migration

Thanks to Alaric Bond for contributing to this post.