This is Armillaria ostoyae. It's larger than the 110-foot long, 200-ton blue whale and occupies about 2,384 acres (almost 4 square miles!) deep in Oregon's Blue Mountains. It's not only the world's largest mushroom, it's also the world's largest known organism.
The fungus is believed to be around 2,400 years old, but could possibly be even older, maybe as old as 8,650 years. That would make it one of the oldest living organisms. It grows along tree roots and is quite resilient, having grown for over a millennia.
"A team of forestry scientists discovered the giant after setting out to map the population of this pathogenic fungus in eastern Oregon. The team paired fungal samples in petri dishes to see if they fused (see photo below), a sign that they were from the same genetic individual, and used DNA fingerprinting to determine where one individual fungus ended."
In 1992, Myron Smith, a botanist from the University of Toronto discovered the Armillaria bulbosa near Crystal Falls, Michigan. It was a ginormous 37-acre fungus. There's even an annual "fungus fest" that's celebrated at the town.
The Armillaria fungae is a network. Collectively its size and shape are indefinite.
[M]icrobiologists developed a new way to tell an individual apart from a group of closely related siblings using a battery of molecular genetic techniques. The major test compared fungal genes for telltale signs of inbreeding, where heterozygous strips of DNA become homozygous. That's when they realized they had struck it big. The individual Armillaria bulbosa they found weighed over 100 tons (90.7 metric tons) and was roughly 1,500 years old.
However, this image of a monster mushroom towering over awed-onlookers is a digital manipulation: