What is the largest living thing? Not animal, but living organism?

The biggest living thing is not, in fact, an animal at all, but a plant. The Quaking Aspen forest holds the record.

The Quaking Aspen sends out underground shoots that re-emerge elsewhere as new trunks. Quaking Aspen groves have been measured at two hundred acres wide, almost seven thousand tons worth of Aspen, all from the same plant! That makes the Quaking Aspen grove the largest living organism on earth.

You might be wondering, if the connections are underground, how do we know any of this is true?

By taking genetic samples from a grove of trees, you can determine which are from shoots of the same underground root system and which are separate trees. Nothing else comes close, either in mass or area, to the Quaking Aspen.