bild

Coral islands in Australia's Great Barrier Reef, satellite image. These islands were formed millions of years ago by coral. Coral is made of many primitive sea animals which leave behind a limestone skeleton after their death. The islands are surrounded by lagoons, enclosed by coral reefs. The Great Barrier Reef is a vast ecosystem made up of similar islands and coral reefs, stretching for 2000 kilometres along Australia's north-east coast. (KEYSTONE/SCIENCE PHOTO LIBRARY/PLANETOBSERVER)