Why did I put the underworld inside Yggdrasil? Believe me: I tried several times to make realistic maps of the nine lands of Norse mythology. For my research, I used both eddas and maps from other researchers.

One famous map is the slanted 3-tier, where the middle tier intersects with the one above and below to make a great "Z". I also tried the famous map where each great root of Yggdrasil reaches to a whole new land. I tried starting one map with a river in a narrow chasm between a frozen land of ice and a raging land of fire, and melted the rest of the lands out of them. None of these ideas made a cosmology that made any sense ... or would work in my fantasy novels.

Traditional Norse descriptions of the routes that the gods traveled, how they got to one place from another, disagree on the relationships between lands. Many historians have tried to map Yggdrasil, but like me, they failed to reconcile everything. (You might begin to suspect that the original tellers of these tales had never actually been there.)

Yggdrasil was the one constant in all of my plans, and the ruin of several of my world-concepts for VIKINGS!. When I began to conceive of the Norse cosmos, I decided to start with the great tree alone. But my desire was to make the lands around Yggdrasil into a simple concept; adding lands so distant that they could only be seen from the top-most branches of the great tree would have made traveling between those lands impossible, each journey taking months instead of days. Introducing something else that never existed in Norse mythology, like a flying cart, a magic carpet, or a friendly dragon that they could ride on, would have made matters worse; I didn't want to introduce a plot device that would obviously be nothing more than a 'convenience'. Unlike on Earth, I wanted everything on the far side of the 'crack in the world' to be totally Norse. For the last book, Quest for Valhalla, I wanted a cosmology for a fast-paced adventure; tilted worlds and vast distances wouldn't work.

So, where to put these vast lands in a world where the only cosmological constant is a giant tree? Where else? I put the lands in its branches. So, where would that put the Underground? Inside the tree! I admit that it is a unique interpretation, but it is just as unrealistic as several other's maps/cosmologies. Also, the description in one of the eddas is that the three roots of Yggdrasil led to different levels of the Norselands, I made due to a knothole on each root; the companions followed one knothole to Niflhiem. I purposefully didn't identify the knotholes on the other roots, leaving them open for other stories.

Of course, it is written that one of these roots leads to Midgard (Earth), so I had to give the companions a route to get back home, but I didn't have to make it one that mortals could travel. Again, this was my interpretation; to make them cross all of the lands of Norse mythology partly to find a different route to get back home.

I am happy to discuss alternatives, but I think that my cosmology works for The VIKINGS! Trilogy, which was all that I required. It isn't perfect (I am also a researcher), but then, ask an archeologist: the truth really isn't known.