Sky Cannon

The Sky Cannon is a rusted, ancient Oocca cannon found in Twilight Princess beneath Renado's house in Kakariko Village. The Sky Cannon, when properly assembled and repaired, will send the "messenger who holds the Dominion Rod" to the City in the Sky, via Fyer's platform on Lake Hylia. The Sky Cannon can be obtained only after Link has possession of the Dominion Rod - the Owl Statue that guards the room where the Sky Cannon rests can only be moved by activating it with the Dominion Rod, similar to statues found in the Temple of Time dungeon.

Discovery and Repairs to the Sky Cannon
After moving the Owl Statue with Shad in Renado's basement and finding the dilapidated, broken Sky Cannon in the subsequent room, Shad tells Link his best bet to get it repaired would be to take it to a cannon expert. Link goes to speak with Fyer, who immediately recognizes the huge, ancient cannon. Fyer says that he can repair the Sky Cannon for him, asking for an advance payment of an exhausting 300 rupees for the job. Fyer then says he will “only be a minute”, but in fact takes several days.

After the repairs are complete, Fyer warns Link that although the Sky Cannon looks like junk, it has some “powerful innards” and that Link will be “knocking on the doors of the Heavens". Link simply uses his Clawshot to enter the Cannon, and a rather frantic Ooccoo follows suit, from which the Cannon blasts Link to the City in the Sky, the seventh dungeon of the game.

Even after Link completes the City in the Sky dungeon, the Sky Cannon will shoot him into the heavens whenever he pleases, and its counterpart in the heavens will do the same back to Hyrule.

Trivia

 * A humorous time-lapse video clip of Fyer's meticulous, rampant work on the Sky Cannon can be seen here.