The largest living organism in the world is a fungus in Oregon.
Why it is true
The Armillaria ostoyae covers almost ten square kilometres of underground forest and is thousands of years old, surpassing in size and mass any whale or sequoia.
Source: Forest 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.