Mermaid Key

The is an item in. It is the key to Mermaid's Cave and exists in both time periods, as the Old Mermaid Key for the present and simply the Mermaid Key for the past. Both keys are obtained in a similar fashion.

Old Mermaid Key
To obtain the Old Mermaid Key, Link must first obtain the Brother Emblem from the Graceful Goron by successfully completing the Goron Dance in either the present or the past. Obtaining the Brother Emblem makes Link an honorary Goron, allowing him to bypass the Goron guarding the stairs to the upper reaches of Rolling Ridge. Next, Link must complete the Target Carts mini-game with a perfect score in order to receive the Rock Brisket. This item is given to the very same Goron Guard, who happens to be very hungry. In return, he will give Link the Goron Vase. Back in the past, the Goron Guard (who is the ancestor of the one in the present), ironically, would like the vase to use as an heirloom to teach his descendants the value of life. In return, Link will be given the Goronade. Finally, after giving the Goronade to the Goron in charge of the Big Bang Game, Link will gain the opportunity to play for the Old Mermaid Key. Once he wins, the key is his.

Mermaid Key
The Mermaid Key is also obtained through a side-quest. Where the Target Carts mini-game is in the present, in the past there is the Goron Shooting Gallery, much like the shooting gallery in Lynna Village. If Link achieves a score of 100 points or more, he will receive the Lava Juice as a prize. Next, in a cave near the summit of Rolling Ridge, Link will meet a Goron who happens to be an acquaintance of the Graceful Goron. If Link already has the old version of the key, the Goron will tell him that it is the Graceful Goron who owns the Mermaid Key. In exchange for the Lava Juice, the Goron will offer to introduce Link to the Graceful Goron with the Letter of Introduction. Once it is shown to the Graceful Goron in the past, he will agree to give Link the Mermaid Key, but only if Link proves 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.