The world's biggest living organism is not the 220 ton Blue Whale. It's a fungus in Oregon, and not only is it the biggest living organism in the world, it may also be the oldest :
Next time you purchase white button mushrooms at the grocery store, just remember, they may be cute and bite-size but they have a relative out west that occupies some 2,384 acres (965 hectares) of soil in Oregon's Blue Mountains. Put another way, this humongous fungus would encompass 1,665 football fields, or nearly four square miles (10 square kilometers) of turf.
The discovery of this giant Armillaria ostoyae in 1998 heralded a new record holder for the title of the world's largest known organism, believed by most to be the 110-foot- (33.5-meter-) long, 200-ton blue whale. Based on its current growth rate, the fungus is estimated to be 2,400 years old but could be as ancient as 8,650 years, which would earn it a place among the oldest living organisms as well.
Armillaria has the unique ability to extend rhizomorphs, flat shoestringlike structures, that bridge gaps between food sources and expand the fungus's sweeping perimeter ever more.
A combination of good genes and a stable environment has allowed this particularly ginormous fungus to continue its creeping existence over the past millennia.
...humongous may be in the nature of things for a fungus. "We think that these things are not very rare," Volk says. "We think that they're in fact normal."