A giant honey mushroom considered a contender for the largest organism on the planet is both much larger and much older than previously thought.

Scientists first studied the enormous fungus, which lives deep underground in a Michigan forest, in 1992.

Then they estimated it was 1,500 years old, and the extensive mass of underground fibres and mushrooms that formed it weighed 100,000kg and stretched 15 hectares.

Returning to the site, the same team used more rigorous testing to estimate the fungus was in fact closer to 2,500 years old.

They also discovered that it weighed 400,000kg and stretched over 70 hectares.

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This makes the enormous honey mushroom, which mostly consists of an underground network of tendrils wrapped around tree roots, heavier than three blue whales.

“I view these estimates as the lower bound… The fungus could actually be much older,” said Professor James Anderson, a biologist at the University of Toronto who undertook both studies.

Their findings were published in the journal Proceedings of the Royal Society B.

While the Michigan fungus is large, it is outclassed by another honey mushroom from Oregon that is even larger.