The first textual sighting of Yggdrasil was in Völuspá which is the first poem of the Poetic Edda. The seeress who is the speaker in Völuspá devotes a stanza to the tree, stanza 19:
I know an ash tree that stands, called Yggdrasil,
A tall tree, sprinkled with white mud;
Thence come the dews that run into valleys,
Forever it stands green over the Urdarbrunn.
A tall tree, sprinkled with white mud;
Thence come the dews that run into valleys,
Forever it stands green over the Urdarbrunn.
The Scandinavians believed that there were níu heimar or "nine worlds" that were part of Yggdrasil. These are said to be as follows:
Ásgarðr, world of the Æsir.
Vanaheimr, world of the Vanir.
Miðgarðr, world of humans.
Muspellheim, world of the primordial element of fire.
Niflheimr, world of the primordial element of ice
Hel, underworld or world of the dead.
Álfheimr, world of the Álfar.
Svartalfheim or Nidavellir, world of the Dvergar.
Jötunheimr, world of the Jötnar.
Vanaheimr, world of the Vanir.
Miðgarðr, world of humans.
Muspellheim, world of the primordial element of fire.
Niflheimr, world of the primordial element of ice
Hel, underworld or world of the dead.
Álfheimr, world of the Álfar.
Svartalfheim or Nidavellir, world of the Dvergar.
Jötunheimr, world of the Jötnar.
Some believe that the Norse myth of Ragnarök also stems from Yggdrasil.
*Some information from- Norse Mythology: A Guide to the Gods, Heroes, Rituals, and Beliefs by John Lindow. Copyright 2001*