In the "Kingdom Hearts" universe, travel between worlds is normally not possible. Worlds are protected from extraterrestrial interference by an invisible shell. When the heart of a world is opened, the shell breaks apart, appearing as a meteor shower. Fragments from the wall are called "Gummi blocks" and are used to make spaceships called "Gummi Ships", which serve as the main mode of travel between the various worlds. Gummi Ships can be shaped into any structure, and the origin of the Gummi Ship material allows for travel to other worlds. Gummi blocks serve different functions, from navigation to offense and defense. Other methods to travel between worlds are the "corridors of darkness" and the "lanes between"—interdimensional pathways through which frequent travel eventually erodes unprotected users' hearts with darkness. These pathways are normally used by Heartless and Nobodies, but have been used by other characters in the series, including Riku and Mickey Mouse.