In Norse mythology, Yggdrasil (from Old Norse Yggdrasill meaning "Ygg's (Odin's) horse" is the world tree. Yggdrasil is attested in the Poetic Edda, compiled in the 13th century from earlier traditional sources; and the Prose Edda, written in the 13th century by Snorri Sturluson. In both sources, Yggdrasil is an immense ash tree that is central and considered very holy. The Aesir go to Yggdrasil daily to hold their courts. The branches of Yggdrasil extend far into the heavens, and the tree is supported by three roots that extend far away into other locations; one to the well Urdarbrunnr in the heavens, one to the spring Hvergelmir, and another to the well Mímisbrunnr. Creatures live within Yggdrasil, including the harts Dainn, Dvalinn, Duneyrr and Durapror, and an unnamed eagle, and the wyrm Nidhoggr.