Yggdrasil, the World Tree, is a massive 'tree' of unknown origin. Whether it is actually a tree in the sense of an oak tree, or whether the mortal mind merely interprets it as a tree is unknown.
Regardless of what it is, it is known that it is possibly the largest living thing in existence, spanning not only a world (its trunk supports the highest mountains -- the Pillars of Heaven -- on the Aetolian world and its root system spreads through the world), but the Planes of Existence themselves. It is said, in fact, that its roots extend through all the Infernal planes, though this may be merely a rumour spread by those with an excess of imagination.