In the twilight world of a chasm on the outer edge of the islands, sea spiders chew deadman’s fingers - a soft coral that’s common at these depths. Sponges and feather stars feast on the microscopic life swept along in the tide. Delicate, slow-growing sea fans angle their growth across the current to increase their catch of food. Below the level where weed can grow, a multicoloured carpet of sea anemones covers the rocks. Even more spectacular - tiny jewel anemones colonise the overhanging edges and vertical faces of these underwater cliffs. Only in the clearest seas will they form such dazzling displays. For those divers that venture beyond the shallow, more recently lost lands of Scilly, to the edge of the rocky plateau on which the archipelago stands, this is an underwater garden of unrivalled brilliance. A feature for which the islands are justly famed. On a smaller scale, the Devonshire cup coral is solitary. Little larger than a thumb-nail, it is only common in the south and west of Britain. Rarer still, and confined to warmer seas, the yellow cup coral is one of Scilly's real living' treasures. Slow growing and long lived, they are more at home in the Mediterranean. Indeed so rich is the undersea life here, it is now defined as a marine park of national importance.