Y is for Yggdrasil

Yggdrasil is the tree of trees, the Ash tree that covered all 9 worlds in Norse Mythology and offered itself up to all life. From it sprang the two humans Lif and Lifthrasir who survived Ragnarok. Odin hung from Yggdrasil when he discovered the runes.

"An ash I know there stands,

Yggdrasill is its name,

a tall tree, showered

with shining loam.

From there come the dews

that drop in the valleys.

It stands forever green over

Urd's well^"

In the poem Grímnismál (where Odin disguised as Grimnir informs Agnar), Odin says that the ash Yggdrasil has three roots that grow in three directions. He details that beneath the first lives Hel, (the ruler of the underworld), under the second live the frost giants in Jotenheim and beneath the third lives mankind in Midgard. A squirrel named Ratatosk must run across Yggdrasil and bring "the eagle's word" from above to Nidhogg the dragon below.