The largest living organism on Earth is a fungus.
Why it is true
Located in Oregon, USA, the 'Humongous Fungus' covers over 9 square kilometers and is thousands of years old.
Source: Biology
Would you rather be asked first?
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.