Located 360 kilometres south of Jakarta, Australia's Christmas Island is home to one of nature's greatest, most colourful and spectacular natural mass migrations. Each year this pristine island's shoreline is covered with an ankle-deep carpet of red crabs. Yet while the frenzy of spawning that ensues should be increasing their numbers, they are actually dropping, as many of the crabs fall victim to the alien yellow crazy ants. The ants release formic acid as they crawl over and around the crabs which, with no defence against the ferocity of the onslaught, become stressed and generally die within a few hours. With constant perils and obstacles in their way, the red crabs are threatened now more than ever before and the ecology of the island is at risk.