Yggdrasil

Yggdrasil is an ash tree and is known as The Tree of Life. The ash is depicted as one of the strongest trees as it is an evergreen, it was said that while the worlds survive the leaves of Yggdrasil would never fall. The branches spread across the worlds and the heaves, with a well beneath each of it's roots.

One of the roots is beneath Niflheim at the site of the main well Hvergelmir, it is from this well that the eleven rivers flow from. This location is a nightmare for all living things; the great hall is made from the bones of poisonous snakes and houses the most poisonous snake to ever exist Nidhogg. Throughout his existence Nidhogg chews at the roots of Yggdrasil in an attempt to destroy the tree.

The second of the great roots reaches out towards the land of the frost-giants. This well is called Mirmir's Well, it is this well that contains great knowledge, it is Mirmir himself who guards the well.

The third is in the heavens and the well located here is known as Urd's Well, the Gods travel to this well every day  in order to hold the courts there. Only two swans live within the well the ancestors of the modern swans. To it's side stands a glorious hall in which the three maidens; Urd (Fate), Verdandi and Skuld reside. They are known to all as the Norns, Urd is from the past, Verdandi the present and Skuld is the future. They choose the destinies of man and weave them into the time stream, giving some longer and shorter lives, others good or bad lives.

Each day the Norns take water from the well and sprinkle it over the branches of the tree to provide it with nourishment to ensure it's survival. This sprinkling of water forms the due upon Midgard and the other eight realms. It is upon Yggdrasil that the nine realms reside.

(Image sourced from; http://www.elfwood.com/~aolson/Yggdrasil-The-World-Tree-in-Norse-Mythology.2531682.html)