Yggdrasil. Poul Anderson realizes it as a physical place in War Of The Gods (New York, 1997) and in Operation Luna (New York, 2000).
In War..., Odin and Loki climb over twisting and swaying branches and through "...caverns of leaf..." (p. 82) to the branch from which Odin hangs himself. He is sacrificed to himself. Loki wounds him with his own spear. We recognize the central act of Christianity in a pagan myth.
In ...Luna, Virginia (Ginny) Matuchek (a witch), Edgar (a raven, Ginny's familiar) and Fjalar (a dwarf) see:
light from an unseen sky;
a root like a cliff rising from the soil;
rich green moss;
a trunk too broad to show curvature, fading in both directions, rearing as if forever;
a gleam brighter than gold;
the mouth of a tunnel;
warmth, life like a tide, strength, abidingness;
a ghost of the wind between the worlds;
roots deeper than death, crown among the stars.
The wind is always with us.