Coral reef in the tidal zone of Green Island, Queensland, Australia, as part of the Great Barrier Reef. Coral common in tropical seas, such as this, is made of calcium carbonate secreted by marine organisms (invertebrates) living together in colonies. A reef is a ridge or island chain that projects above the surface of the sea during tidal cycles. A barrier reef is one which develops offshore so that a deep lagoon (as seen here) is formed between the reef and the mainland coast- line (background). Stretching 2000km long and 50- 150km offshore the Great Barrier Reef is the longest of its kind, and the largest accumulation of coral known.