This is the stuff of nightmares for some people, but is an amazingly beautiful natural phenomenon for others. Every year on Christmas Island in the Pacific Ocean, 40 million red crabs leave the forests at exactly the same time, heading for the ocean for spawn. The migration is so huge that it literally stops traffic on the tiny 52 square mile island, as the crabs flood across roads trying to get to the sea. In the past many were crushed by vehicles and accidents were common as the crabs’ exoskeletons burst tyres. Today special fences funnel the crabs into tunnels so they can cross the roads safely. A short while after the migration something beautiful happens — millions of newly spawned crabs head back up on to the land the same way their parents came, to live their lives in the forests.