https://www.science.org/content/article/humongous-fungus-almost-big-mall-america
‘But even with its new size estimate, the Michigan fungus has already been eclipsed by a different Armillaria in Oregon, discovered in 1998. That one, now the largest organism in the world, may be more than 8000 years old and covers more than 770 hectares.’
1 hectare =2.471 acres