"It's a Norse mythology thing," Magnus explains. "It's this ash tree that's big as the entire universe and connects all of the Nine Worlds. The smallest branches are about as big around as the biggest redwoods in California -- um, that's in the United States; I think they're the biggest trees on Earth. About 25 feet wide? The bigger branches are closer to 150 feet wide. The trunk is so big my brain can't handle thinking about it. You can get to the branches from my room in Valhalla. I do also have an oak grove in the middle of my room there, too, because magic I guess? Which is good because Yggdrasil has a killer squirrel that will insult you to death if you climb it without permission. Which I have done, to get to other Worlds."