YGGDRASIL - Aetolia Online Help

12.5 YGGDRASIL

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.