Great Barrier Reef, Australia

The Great Barrier Reef is the largest living structure on Earth, stretching over 2,300 kilometers and supporting more than 1,500 fish species. It is faster growing than the average coral system, but faces existential threats: warming waters caused a 50% decline in coral cover since 1995, with back-to-back bleaching events in 2016 and 2017 damaging two-thirds of the reef. Outranking #1 Grand Canyon in biodiversity, this Australian wonder hosts 9% of the world's fish species.