"Then said Gangleri: "Where is the chief abode or holy place of the gods?""
"Hárr answered: 'That is at the Ash of Yggdrasill; there the gods must give judgment everyday.""
"Then Gangleri asked: "What is to be said concerning that place?""
"Then said Jafnhárr: "The Ash is greatest of all trees and best: its limbs spread out over all the world and stand above heaven."
"Three roots of the tree uphold it and stand exceeding broad:"
"one is among the Æsir; another among the Rime-Giants, in that place where aforetime was the Yawning Void;"
"the third stands over Niflheim,"
"and under that root is Hvergelmir, and Nídhöggr gnaws the root from below."
"But under that root which turns toward the Rime-Giants is Mímir's Well, wherein wisdom and understanding are stored;"
"and he is called Mímir, who keeps the well."
"He is full of ancient lore, since he drinks of the well from the Gjallar-Horn."
"Thither came Allfather and craved one drink of the well;"
"but he got it not until he had laid his eye in pledge."
Source: Gylfaginning ch 15