FOR CENTURIES THE CREATURE HAS been munching, munching, munching. Hidden in the depths of the earth, it has grown bigger and bigger with every bite; now it is the largest known living thing on the planet. Vaster than the Vatican, the ravenous 1,500-acre monstrosity is slouching out of its birthplace in the forests of Washington State. Could it be headed toward (gulp!) Seattle? But don't scan the horizon for a scaly creature from 1,000 fathoms. Look for a fungus among us.

Just because Armillaria ostoyae is vegetable rather than animal doesn't mean it can't be pestilential. Unlike a smaller 38-acre cousin discovered in Michigan last year, A.o. has the nasty habit of feasting on the roots of live trees. "It spreads out like a ringworm," says Ken Russell, a Washington forest pathologist, "You don't really see the fungus. You see the dead and dying trees."

For the last 26 years, Russell has been battling the 2½-square-mile beast, which dwells one to three feet under a pine forest near Mount Adams, southeast of Seattle. He has ripped out stumps with bulldozers and shovels to keep it from spreading from root to root. In the fall he watches for mushrooms to pop up—a sure sign that beneath lies the shapeless behemoth. And forget about eating the eater. "It's edible," says Russell, "but it's fairly ho-hum and might make some people sick."

For the moment, Russell has kept Armillaria in check. However, if the "creepy crud," as one of his assistants calls it, isn't continually tracked, it would, says Russell, "quite simply grow until it runs out of trees." There may be other fungi lurking. "It's kind of like fish," says Russell. "You always know there's a bigger one out there somewhere." And you thought it was safe to go back into the forest.