I won't tell you where Yggdrasil comes from. The tree is just there. It can't be spotted from the earth, as it is invisable for the mortal, but without the enormous World-tree, everything would dissolve and explode into the eterity...

Yggdrasil is the biggest and most beautiful of all trees. Its branches are all over the nine worlds and all over the sky. The tree has three roots, to keep it up. One root is into Asgård (The home of the gods), the other one is into Jotunheim (The home of the giants) and the last root is into Nifelheim (The world of snow, ice and fog).
There are three wells which give water to the three roots. The one in Asgård is called Urd, after the most powerful of the three sisters who guard it. The sisters are relatives with the moon, and their influence over mens' life is without limit. They are called the Norns, and they sit around the well, crouching down and wrapped in their grey shawls. Nobody have ever seen their faces, as they are hidden in shadows. Nobody knows if they are young and beautiful or old and wrinkly. Their well is especially sacred and they mix its water with the mud from its sides into a dough, which they spread over the root, every morning. This neutralizes the powers of the evil and store the root. Their work prevents the tree from rottening. The sisters came originally from Jotunheim (The home of the giants). They seemed to be sisters of Nör, the father of Natt (Night). Therefor the sisters were related to the darkness and the moon.
Every morning, after taking care of the root, they started to spin. It wasn't an ordinary thread, it was the thread of life. Not only the newborn humans and the gods, but the destiny of the entire universe, hang upon it. As it is the Norns who measures the time and controls the past, this very moment and the future, as they are spinning. Their names mean; Urd (It-which-has-been), Verandi (It-which-is) and Skuld (It-which-will-be). The Norns means the Destinys.
The well of Urd has also given life to two white swans, from which all swans descend.

Yggdrasil's second root is into Jotunheim. The well there is the well of Mimer. Mimer was a god once, but now, only his head remains and is being kept alive by magic herbs. His head is full of wisdom, because he drinks from the well every day. The head of Mimer is still able to speak, but only if Oden, the forefather of all gods, speak to it. This is luckily, because in the end of time, Oden has got to speak to Mimer if he wants to know how to save himself, the Æsirs and his world.

Down in Nifelheim (The land of snow, fog and ice), is the third well; Hvergelmer. The third root hangs above the boiling well and its venomous steam. In Hvergelmer, lives the terrible dragon Nidhögg (The terrible biter). His terrible mounth is gnawing at the root and hurts it.
Together with Nidhögg, in Hvergelmer, are many snakes which breathe venomous-clouds on the suffering root. Some of the snakes names are Gifttand (Venomous-tooth), Gravulven (Grave-wolf) and Rotgnagaren (Root-gnawer).
Then there are four enormous deers, which eats from Yggdrasil's top. In the highest bransh of Yggdrasil, an eagle is sitting. The beak is so big that a hawk is sitting on it. The eagle is very wise but sullen. A squirrel called Ratatosk is scampering between Nidhögg and the eagle to exchange abuses.

The story about how the tree got the name Yggdrasil is frightening. Yggdrasil means Ygg's horse, and Ygg is one of Oden's many by-names. Maybe you have heard, that the gallows tree sometimes is called the hanged man's horse? Well, this was how it happened:
Oden wanted to learn the secret of the runes, these magic signs from which the art of writing grew. But such a secret demanded a great sacriface from the one who wanted it and Oden had to hang by his neck for nine days and nine nights over the endless pit. He also had a spear which stung him. While he was hanging there, he carefully spyed down into the gulf and caught the runes that came by him by the winds of the gulf. After this torture he got to know nine powerful secrets about the runes and then he offered his services to the other gods and the people.
Yggdrasil's name before this happened was the tree of Mimer, which tells us how great Mimer once had been...