Odin is known to be one-eyed because of a trade between Mimir and Odin. When Odin asked Mimir for some water in the well, Mimir declined him of giving any water unless Odin gave him an eye. Surprisingly, Odin took one of his eyes out and gave it to Mimir for water. The trade was made and Odin was very happy for the water.
But what did Mimir do with Odin's eye? The source tells nothing about the use for Odin's eye or what happens to it.