In Norse Lore, Yggdrasil, the world tree is a giant Ash the connects the 9 worlds, Homes of men, gods, dwarves, and giants. At the base of one of the roots of the tree is a spring that feeds a deep well.
The god Mimir lives at this well. He drinks from it every day and it contains all the knowledge of the universe. Odin visits Mimir often for counsel but wants to have his own knowledge from the well.
Mimir allows this, only if Odin trades his eye for a sip. Odin agrees and now his eye will forever be a part of that spring so that all will see his sacrifice to Mimir’s Well.