Even if you can’t pronounce it, you have likely heard of the great World Tree called Yggdrasil. It’s part of Norse mythology, and is literally the central part as this ash tree is believed to grow up through of all the various worlds, connecting them.
I’ve always envisioned this tree with its roots in the underworld (Niflheim) and its highest branches in the realm of the Gods (Asgard). Well, that’s not really how it’s laid out after all. There are 3 roots to Yggdrasil, and under each of the roots is one of the mythological worlds: Asgard, Niflheim (or Hel), and Jotenheim. The trunk of the tree runs through the realm of man, Midgard.