In 1975 an army of gigantic crabs, the result of an underwater nuclear experiment, attacked the Welsh coastline. The battle was bloody, many lives were lost until the crustacean invaders were defeated.
Over the ensuing years they turned up in the oceans of the World with further terrible slaughter of humans. Finally, though, it was believed that these monsters from the deep had been eradicated. Only memories of their invasions of land remained with the older inhabitants, tales of their depredations on mankind were whispered but often ridiculed by the modern generations.
Until a few of the survivors returned to the Welsh coast and began breeding secretly in a maze of caverns beneath the cliffs, preparing for a further attack on mankind.