Heart Reef, part of Australia's Great Barrier Reef.
Credit: Mornee Sherry | Shutterstock
The Great Barrier Reef is the world's largest coral reef system (and it's largest living structure ), comprising nearly 3,000 individual reefs and 900 islands off the eastern coast of Australia.
One of these thousands of reefs is Heart Reef, so named for its shape. It is located in the Whitsunday Islands and must be viewed from the air as the area is protected and snorkelers and scuba divers are not allowed to enter.
Like other coral reefs around the world, the Great Barrier Reef, and the creatures that depend on it, is threatened by human activities such as overfishing, pollution from river runoff and the rise of ocean temperatures linked to climate change.