A FUNGUS! Thats right. Yes, that stuff that appears out of nowhere when your food is over the expiration date. Not a blue whale, not a sequoia tree, but a honey fungus.
It lives in the Blue mountains in oregon, and lives on 2,400 (965 hectares) acres of soil, or 10,000 square meters. That is as big as 1,665 football fields. DAMN. That is actually bigger than the two smallest countries in the world combined.
Honey fungus is a common name given to multiple different species of fungi in the genus Armillaria. The fungus is a giant, but not so very friendly. Honey fungi are parasitic and kill a wide range of trees and woody plant species. Therefore they earned the nickname "Gardeners Nightmare".
Fungi grow by expanding tubular filaments called hyphae, which form a web-like network. In this network resources are transported, as well as water, organic molecules and other important materials for the fungus. Resources are collected on the surface, where the mushrooms are. The mushrooms of the Armilaria are edible yellow-brown mushrooms.
Mushrooms are the fruiting bodies of fungi. Imagine an apple tree entirely underground, but just the apples are above ground. Many people believe that mushrooms are only there to procreate but they actually have another extremely important function.
Mushrooms digest food. They actually excrete digesting enzymes, which breaks leafs on the ground apart for example. The small particles created by these enzymes are then taken up by the fungus. So basically, mushrooms are the mouths of fungi.
There has been a few questions about the defenition of a single organism. Now it is defined as a group of genetically identical cells that can communicate and have a common purpose. This fungus meets these requirements, and so it currently holds the record as the largest KNOWN living organism. I repeat KNOWN, because who knows what more is out there.
Source: http://www.iflscience.com/plants-and-animals/meet-worlds-largest-living-organism/