In Norse mythology, the secret knowledge held by the Well of Mimir pertains to wisdom and understanding. Mimir, known as a wise being, guards this well, and Odin sacrifices his eye to drink from it, gaining profound insights into the cosmos and fate. This act signifies the lengths to which one might go for knowledge, embodying the importance of wisdom in Norse culture. The well itself represents not only knowledge but also the deep connections between wisdom and the supernatural.