Great Barrier Reef, Queensland, Australia

© AWL Images/Danita Delimon

The largest living organism on Earth

Today marks the second-ever World Reef Day, an observance created to help focus global attention on the deteriorating health of our planet's vital reef systems, and also to help marshal efforts to help them recover. Why should we care about the health of ecosystems such as the Great Barrier Reef, seen in this photo? Because reefs are an easy indicator of how healthy our oceans are. And right now, reefs aren't doing too well. What can we do? Learn about reefs—not just the dangers that put healthy reefs at risk, but why reefs are such a critical part of Earth's greater ecosystems.