The biggest living thing on Earth is a Fungus

In Oregon’s Malheur National Forest, the biggest living organism on Earth is a fungus. (Oregon Encyclopedia)

It’s one single, genetically connected individual of the honey mushroom fungus (Armillaria ostoyae), known as the Humongous Fungus. What makes it hard to wrap your head around is that most of it is hidden. The organism is mainly an underground mycelium network running through soil and tree roots. The mushrooms people notice are just the fruiting bodies it produces, not the main body. (Oregon Encyclopedia; Oregon Public Broadcasting, Oregon Field Guide)

This one organism has been documented at about 2,385 acres (roughly 3.7 square miles). It has also been described as potentially up to about 8,650 years old. That is a living network, holding ground under a whole forest. (Oregon Encyclopedia)