The largest structure built by living organisms is the Great Barrier Reef. Located off the coast of Queensland, Australia, it is composed of over 2,900 individual reefs and stretches more than 1,400 miles. This ecosystem is primarily built by coral polyps, tiny marine animals that secrete calcium carbonate to form the reef structure. The Great Barrier Reef is home to thousands of species, including fish, mollusks, and sea turtles, making it one of the most biodiverse ecosystems on the planet. Its vast size and complexity have significant ecological importance, providing habitat and contributing to marine biodiversity.