In June each year, an intriguing phenomenon occurs in the waters off Melbourne, Australia. A swarm, some 100,000 strong, of great spider crabs covers a stretch of seafloor. They each measure about 20 centimeters wide. Their legs can extend up to 40 centimeters. When faced by predators many of this sprawling phalanx climb upon each other creating towers of writhing crabs over 2 meters high. Using the latest equipment and the help of one scientist, all the details of this mysterious mass gathering were successfully filmed. For the first time, the reason behind this gathering has been explained. This program captures the drama of crabs engaging in fierce battles with predators and risking their lives, all for the purpose of congregating in a single mass.