World Tree

The «World Tree» (世界树), also known as «Yggdrasil», is the massive tree growing at the center of ALfheim.

Norse Mythology
In Norse mythology, Yggdrasil is an immense ash tree at the center of Norse cosmos, connecting Niflheimr (or Niflheim; the realm of ice and mists), Jötunheimr (or Jotunheim; the homeworld of the giants (Jötnar)), Álfheimr (or Alfheim; the homeworld of the elves (Álfar)), Miðgarðr (or Midgard; the realm of humans (Menn)), Ásgarðr (or Asgard; the home of the Æsir gods), Vanaheimr (the home of the Vanir gods), Hel (the world of the dead), Svartálfaheimr (the homeworld of the dwarves or dark elves) and Múspellsheimr (or Muspelheim; the realm of fire) - the nine worlds in Norse Cosmology. The name "Yggdrasil" in Norse mythology means "The ash tree of the horse of Odin", where "drsail" means "horse" and Ygg means "The Terrible One", another name for Odin.

The tree grows out of the Well of Urðr (Urðarbrunnr), where three female divine beings, Urðr (past), Verðandi (present) and Skuld (necessity), collectively called the Norns, live and keep the tree watered. There the Norns also shape destiny by carving runes into the trunk of Yggdrasil or by weaving destiny like a web or tapestry. However, time in Norse mythology is considered as cyclical, instead of linear, and the destiny set by the Norns is not final and can still be changed by the individual agency of all living beings. Aside from the Well of Urðr, two supporting roots reach out to two more wells, Hvergelmir in Niflheimr, where the wyrm Níðhöggr gnaws at the root of the tree, and Mímisbrunnr in Jötunheimr.

ALfheim Mythology
It was said that the first race to reach the legendary city, floating above Yggdrasil, and have an audience with Fairy King Oberon, who lived there, would be reborn into true fairies, the ALF. The hover limit imposed by the system would be eliminated and unlimited flight would become possible. Along with it, the successful race would be capable of becoming rulers of the infinite sky. It was said to be possible to enter this floating city through a dome at the root of Yggdrasil.

However, Kirito later discovers that there is no city above the World Tree. Also, Yui claimed that the number of guardians that defended the World Tree was too high, meaning that the Grand Quest was, in fact, never intended to be possible to be completed. This is further backed when Kirito finds that the gate at the top of the tree couldn't be opened by players as it was locked by administrator codes rather than a quest flag.

Fairy Dance Arc
Kirito attempts to climb the World Tree on two occasions to rescue the captive Asuna from Fairy King Oberon. The first, a failed solo attempt and the second with Leafa, Recon, and the Sylph and Cait Sith raid squadrons which was successful. However once Kirito reached the gate he found it locked by administrative codes, Yui determined that the gate was never intended to be opened by players. However, Kirito is able to open it by having Yui use the coding from the admin card Asuna dropped from her cage.

Gallery
Árbol del Mundo Yggdrasil Иггдрасиль World Tree