This event takes place twice a year for a week or two between April and July. First, the adult black crabs descend to the beaches and lay their eggs, before returning to the mountainside. Then, several weeks later, the juvenile crabs leave the sea and follow in their path. Roads usually close to allow safe crossing for the crabs.
Getting around the island is very tough at this time; the result is always utter paralysis, but it's fascinating. The crabs are around at other times of the year, too, so keep a look out.