The biggest living thing lives under a forest

In the Blue Mountains of Oregon, in the United States, a single fungus spreads across more than 900 hectares: almost ten square kilometres of ground. It's an Armillaria, the so-called honey fungus, and it's considered the largest living organism on Earth.

Almost all of it is invisible. It lives underground as a web of white threads, the mycelium, that works its way between tree roots and slowly eats them. Only in autumn does it show itself: honey-coloured mushrooms sprout and give away the hidden giant.

Estimates give it at least 2,400 years, and perhaps far more. All that time growing in silence, killing trees in no hurry.

You step into a forest clearing and think there's nothing there. Beneath you pulses the most far-reaching living thing we know of.