What is the largest living organism by weight?

The largest living organism by weight is a fungal colony of Armillaria ostoyae in Oregon, USA, covering an area of over 2,400 acres (970 hectares). This honey fungus grows underground through mycelial networks, and although it appears as a collection of individual fungi, it is genetically a single organism. Its massive size is due to the extensive network of underground mycelium, making it not only the largest by weight but also one of the oldest living organisms, estimated to be thousands of years old.