The QI Book of General Ignorance

Here's another quite interesting passage from the QI Book of General Ignorance:

Q: What's the largest living thing?

A: It's a mushroom.
         And it's not even a particularly rare one.   You've probably got the honey fungus (Armillaria ostoyae) in your garden, growing on a dead tree-stump.  
        For your sake, let's hope it doesn't reach the size of the largest recorded speciman, in Malheur National Forest in Oregon.   It covers 890 hectares (2,200 acres) and is between 2,000 and 8,000 years old.   Most of it is underground in the form of a massive mat of tentacle-like white mycelia (the mushroom's equivalent of roots).   These spread along tree roots, killing the trees and peeping up through the soil occasionally as innocent-looking clumps of honey mushrooms.  
      The giant honey fungus of Oregon was originally thought to grow in separate clusters throughout the forest, but reseatchers have now confirmed it is the world's single biggest organism, connected under the soil.