The Great Barrier Reef is the largest living structure on Earth.
Why it is true
Built by billions of tiny coral polyps, this massive ecosystem stretches over 2,300 kilometers and is visible from space.
Source: Biology
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This is the reading side of the site, so the answer is in the sentence above — that is the whole point of it. Test yourself works the other way round: the question comes on its own, and the fact and the explanation only appear once you have answered.