The scientists first reported finding it in 1992: a giant mushroom that weighed as much as a blue whale and sprawled across more than 30 acres of forest in Michigan’s upper peninsula. It wasn’t some Alice-in-Wonderland-type toadstool but a 1,500-year-old parasitic mold, with growing tentacles that foraged beneath the soil for roots and decaying wood to devour.

Nearly 30 years later, the same scientists — using new technology for genetic analysis — wanted to know whether they had properly measured this unusual example of fungal life.

“We made this outlandish prediction that the fungus is more than 1,000 years old,” said James Anderson, now a retired mycologist and emeritus professor at the University of Toronto. “And so an obvious outcome of that, is after three decades, it ought still be there, and if not, we’d have some explaining to do.”

Recently they published what they uncovered in Proceedings of the Royal Society B: Biological Sciences. Their original humongous fungus, Armillaria gallica, is even older and bigger than first estimated: the 2,500-year-old parasite spreads across 180 acres of forest. And its genome harbors a mysterious survival strategy: an extremely low mutation rate.