http://www.npr.org/blogs/krulwich/2014/05/08/310259300/a-question-of-biggitude-what-s-the-largest-creature-on-earth
Its an interesting article. It's basically a forest of trees that is all one large root cluster and the other is a giant root cluster of mushrooms that seems to be more of a root plant than actual mushrooms on the ground.
And here is a youtube video explaining it: