Humongous Fungus

Armillaria ostoyae, also called "Humongous Fungus", is a species of fungus forming the largest known living organisms on earth.

Sure, blue whales are the largest living animals that ever existed, but scientists also consider interconnected fungal colonies a single organism (because they are genetically identical).

The largest known colony is located in the Malheur National Forest in Oregon. It is estimated to be 2,400 years old, covers 8.8 square kilometers, and weigh as much as 605 tons.