At the center of the Norse spiritual cosmos is an ash tree, Yggdrasil (pronounced “IG-druh-sill”; Old Norse Askr Yggdrasils), which grows out of the Well of Urd (Old Norse Urðarbrunnr). The Nine Worlds are held in the branches and roots of the tree. The name Askr Yggdrasils probably strikes most modern people as being awkwardly complex. It means “the ash tree of the horse of Yggr.” Yggr means “The Terrible One,” and is a byname of Odin. The horse of Odin is Sleipnir. This may seem like a puzzling name for a tree, but it makes sense when one considers that the tree as a means of transportation between worlds is a common theme in Eurasian shamanism. Odin rides Sleipnir up and down Yggdrasil’s trunk and through its branches on his frequent journeys throughout the Nine Worlds. “Urd” (pronounced “URD”; Old Norse Urðr, Old English Wyrd) means “destiny.” The Well of Urd could therefore just as aptly be called the Well of Destiny.
–from Norse Mythology for Smart People by Daniel McCoy
The tree of life. The tree of the knowledge of good and evil. The tree of Odin’s horse. All images of trees at the center of the world. Many ancient cultures include a central tree with fruit conveying immortality or wisdom or other desirable qualities.
In Persian and Zoroastrian traditions, there is Gaokerena, threatened by a frog and guarded by kar fish. The juice made from its fruit conveys immortality.
In the Kingdom of Ararat, carvings of the tree of life appear on the walls of fortresses.
The first book of the Bible, Genesis, refers to both the tree of life and the tree of knowledge of good and evil. The serpent tricked Eve into tasting the fruit of the tree of knowledge of good and evil, and then Eve convinced Adam he too should eat the fruit. Their punishment was to be expelled from the Garden of Eden where the tree of life remained, guarded by angels with flaming swords.
Buddha sat under a sacred fig tree, also known as the Bo or Bodhi tree, when he attained enlightenment.
In Chinese mythology, the tree of life is depicted with a dragon and a phoenix. Every three thousand years, the tree produces a fruit which when eaten gives the eater immortality.
In Islam, the Garden of Eden contains the tree of immortality, and it is this tree that God forbade Adam and Eve from eating the fruit. In this version as well, the serpent seduced Adam and Eve to eat from the tree, disobeying God.
World trees were associated with pre-Columbian Mesoamerican cosmologies where the trees connect the four directions as well as the underworld and sky with the physical world
Native American Indian tribes often include tales involving trees, such as the Grandmother Cedar tales of the Sammish and Ojibwa tribes.
With all those other societies including stories of sacred trees to explain creation, the desire for immortality, the consequences of disobeying God, and other important life lessons, there should be no surprise to learn that a tree–perhaps an ash, perhaps a yew–holds together the mythological cosmos of the Norse and Germanic ancient culture.
Yggdrasil connects the nine levels of Norse cosmology:
- Niflheim, the primordial frozen land;
- Asgard, the land of the Aesir;
- Midgard, the land of humans;
- Jotunheim, the land of the giants;
- Vanaheim, the land of the Vanir;
- Alfheim, the land of the elves;
- Svartalfheim, the land of the dwarves;
- Helheim, the underworld;
- Muspelheim, the primordial flaming land
Odin travels to all levels of the cosmos on his horse, Sleipnir. Not all creatures of the cosmology can travel beyond their home levels, which explains why so many of the tales involve the Aesir and Vanir traveling to the homes of the elves, the dwarves, and the giants, but not often the other way around.
The well of Urd, guarded by the three Norns, rests at the base of Yggdrasil. Other creatures live among its branches and roots, including an eagle that lives in the top branches, snakes and dragons that gnaw on its roots, a squirrel named Ratatosk who carries messages between the snakes and the eagle, and four deer named Dain, Dvalin, Duneyr, and Dyrathror who nibble at the highest levels. 
 Simek, Rudolf. 1993. Dictionary of Northern Mythology. Translated by Angela Hall. p. 375.
 Eliade, Mircea. 1964. Shamanism: Archaic Techniques of Ecstasy. Translated by Willard Trask. p. 37.
 The Poetic Edda. Grímnismál, stanzas 32-34.