In Norse mythology, Mímir's Well or Mímisbrunnr (also known as the Well of Mímir, the Well of Knowledge) is one of three wells at the roots of the ash tree Yggdrasil. It was named for Mímir, the god charged with guarding it. The well was fed by a fountain inside it called the fountain of Mímir, which granted wisdom and the power to see the future. Mímir was known as the wisest of the gods; his name translates to "memory."
As told in Ynglinga saga, in his youth, Odin desired the wisdom of Mímir's Well, and visited Mímir. They struck a bargain -- Odin sacrificed one of his eyes in exchange for the privilege of drinking from the well. This wisdom helped him lead the Æsir, but showed him a vision of the destruction of the Æsir at Ragnarök. This knowledge of the future weighed heavily on Odin, who was said never to have smiled again.
https://en.wikipedia.org/wiki/M%C3%ADmir's_Well