In Norse mythology Yggdrasill was a great ash tree located at the center of the universe and joining the nine worlds of Norse cosmology. The trunk of the tree was thought of as forming a vertical axis around which these worlds were situated, with Asgard, the realm of the gods, at the top and the underworld realm of Niflheim at the bottom. Midgard, the world of mortals, was located in the middle and surrounded by Jötunheim, land of giants, both of which were separated by an ocean.