Yggdrasil

Yggdrasil is the axis of the Norse universe. In the Eddic poems Völuspá and Grímnismál, and in Snorri's Gylfaginning, it is a vast ash whose limbs spread over the whole world. Its three roots are described two ways. Grímnismál says Hel lives under one, the frost giants under the second and human beings under the third. Gylfaginning sets a well or spring under each: the well of Urðr, where the gods hold court, under the root that stands among the Æsir in heaven; Mímir's well of wisdom under the root among the frost giants; and Hvergelmir under the root over Niflheim.
It teems with creatures: an eagle sits in its branches, the serpent Níðhǫggr gnaws a root from below, the squirrel Ratatoskr runs slander between them, and four harts browse its leaves. The tree suffers constantly yet endures: a living image of a cosmos under strain.
Its name is the honest surprise. The usual reading is 'Odin's steed': Yggr is a name of Odin ('the terrible one'), drasill means horse. Poets could call the gallows a horse that the hanged man rides, and in Hávamál Odin hangs on a windy tree nine nights, wounded with a spear, to seize the runes. Hávamál never names that tree, so the link is an inference, but if the reading is right the world-tree is named for that ordeal, not for 'life'.
All of this is textual, from 13th-century Iceland. No surviving Viking-Age image is securely identified as Yggdrasil; the concept is preserved in poetry and Snorri's prose, not in a datable carving.