Something really interesting just came up but I never really bothered to say anything. My brother has a tatoo of the Yggdrasil on his chest. And what i learned from really just looking into things that the Yggdrasil also is the pathway between Heaven and Hell. I think that the is the Yggdrasil for 2 reasons.
Yggdrasil's outside doesnt grow. The inside of it is all worlds.
I think that Navidson was another "Odin" if you would.
Odin hung himself to gain knowledge.. Knowledge for Pain.
What did Navidson do?
He wanted to know what was going on no matter what.
There is also a serpent/dragon who lives at the bottom to hurt anyone that comes and puts poison into the tree.
Maybe that was what was attacking them at the bottom?
This book is just huge!