The Armillaria ostoyae fungus in Oregon is one of the largest living organisms in the world.
Why it is true
An underground network of this fungus covers about 9 square kilometres and is thousands of years old. Most of the organism is hidden in the soil, in the form of filaments called hyphae. It is estimated to weigh between 7,500 and 35,000 tonnes.
Source: USDA
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.