Mermaid Key

The Mermaid Key is the key to the sixth Dungeon in The Legend of Zelda: Oracle of Ages. It exists in both time periods, as the Ancient Mermaid Key for the present and simply the Mermaid Key for the past. How to obtain both keys follows:

Ancient Mermaid Key
The first step in this quest is to talk to the dancing Goron, who challenges Link to match his dancing skills. Once this is completed, Link will receive a Brother Emblem. After this, the next challenge is to beat the Crazy Carts Game. Once Link wins the game, he will obtain the Rock Brisket. By giving this to a hungry goron, Link will obtain the Goron Vase. Back in the past, the 'same' Goron wants this vase, and once Link hands it over, he will obtain Goronade. By giving this to the runner of the Big Bang Game, Link will gain the opportunity to play for the Ancient Mermaid Key. Once he wins, the key is his.

Mermaid Key
Where the Crazy Carts is in the present, in the past is a shooting gallery, much like the one in Lynna Village. Once a score of 250+ is achieved, Link will earn the Lava Juice, which another Goron nearby desires. In exchange for the Lava Juice, Link obtains a Letter of Introduction, which is to be given to the Dancing Goron of the past, asking for the Mermaid Key. However, the Dancing Goron decides that Link must prove himself by matching his skills in a dancing contest. Once this is done, Link will be the proud owner of both Mermaid Keys, leaving Mermaid's Cave waiting for him.