Omni-Cannon

The Omni-Cannon is a giant cannon created by Franky, capable of launching people into the stratosphere.

Design
The Omni-Cannon is large enough to fit at least two people into its barrel. The cannon rests on two wheels, presumably allowing it to be moved.

Past
Before the events of One Piece: World Seeker, Franky created the Omni-Cannon, placing it at the top of Jail Island's mountain.

Side Mission: Jewel Mine
Luffy, Usopp, and Jeanne climb the mountain, reaching the Omni-Cannon. Jeanne is interested in it, observing that it can probably be used to reach the Sky Prison, and begins asking Usopp for information about how it was built. However, Usopp deflects the questions, telling her she'd need to ask Franky.

Side Mission: Mystery of the Sky Islands
While investigating the Sky Islands above Jail Island, Law and Luffy each use the Omni-Cannon, landing on Law's Sky Island and Luffy's Sky Island respectively.

For the remainder of One Piece: World Seeker, the Omni-Cannon can be used to access the Sky Islands when it's not connected to the active quest.

Chapter 14: Assembly
Jeanne and the Straw Hat Pirates realize that Isaac has gone to the Sky Prison. Franky states that he can adjust the cannon to send them to the Sky Prison, and they resolve to do that.

Chapter 16: Sky Prison, Ho!
Luffy arrives at the Omni-Cannon, using it to launch himself and Jeanne to the Sky Prison.