Mimisbrunnr

In Norse mythology, Mímisbrunnr (Old Norse "Mímir's well" ) is a well associated with the being Mímir, located beneath the world tree Yggdrasill. Mímisbrunnr 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. The well is located beneath one of three roots of the world tree Yggdrasil, a root that passes into the land of the frost jötnar where the primordial plane of Ginnungagap once existed. In addition, the Prose Edda relates that the water of the well contains much wisdom, and that Óðinn sacrificed one of his eyes to the well in exchange for a drink.

Poetic Edda
In the Poetic Edda poem Völuspá, a völva recounts to Odin that she knows that Odin once placed one of his eyes in Mímisbrunnr as a pledge, and that Mímir drinks from the well every morning:

The above stanza is absent from the Hauksbók manuscript version of the poem. Elsewhere in the poem, the völva mentions a scenario involving the hearing or horn (depending on translation of the Old Norse noun hljóð—bolded for the purpose of illustration) of the god Heimdallr:

Scholar Paul Schach comments that the stanzas in this section of Voluspa are "all very mysterious and obscure, as it was perhaps meant to be". Schach details that "Heimdallar hljóð has aroused much speculation. Snorri seems to have confused this word with gjallarhorn, but there is otherwise no attestation of the use of hljóð in the sense of 'horn' in Icelandic. Various scholars have read this as "hearing" rather than "horn".

Scholar Carolyne Larrington comments that if "hearing" rather than "horn" is understood to appear in this stanza, the stanza indicates that Heimdall, like Odin, has left a body part in the well; his ear. Larrington says that "Odin exchanged one of his eyes for wisdom from Mimir, guardian of the well, while Heimdall seems to have forfeited his ear."

Prose Edda
In chapter 15 of the Prose Edda book Gylfaginning, the enthroned figure High tells Gangleri (described as king Gylfi in disguise) about Yggdrasil. High details that Yggdrasil has three roots. One of these roots reaches to where the primordial space of Ginnungagap once existed and where now the frost jötnar live. High explains that, beneath this root is Mímisbrunnr and that the well contains "wisdom and intelligence" and "the master of the well is called Mimir. He is full of learning because he drinks of the well from the horn Gjallarhorn. All-father went there and asked for a single drink from the well, but he did not get one until he placed his eye as a pledge." After his explanation, High quotes the stanza involving Odin and the well from Völuspá.