Scientists traced the find to a 1988 investigation into clustered conifer deaths; lab pairing tests and genetic fingerprinting later showed samples across the site were genetically identical.
The fungus spreads underground through mycelium and root-like rhizomorphs, invading tree roots, killing hosts over years, then feeding on dead wood while expanding to new trees.
Spread-rate estimates put its age at roughly 2,000 to 8,500 years, making it not only the largest known single organism but also one of the oldest continuously living ones.
Forest managers say it cannot be eradicated, so control efforts focus on planting more resistant tree species and avoiding cutting practices that can speed its spread.