<human>:  When was the Great Barrier Reef declared a World Heritage Site?
<bot>: The Great Barrier Reef was declared a World Heritage Site in 1981. It was added to the list of World Heritage Sites because of its unique and diverse ecosystems, which are home to many species of plants and animals. The Great Barrier Reef is the largest coral reef system in the world, and it is located in the Coral Sea off the coast of Queensland, Australia.