You can do this by leaving an interior through a unsolid wall with the jetpack. Because all interiors in a heaven are loaded at the same time, it is possible to reach another interior when you can trick the game into thinking you haven't left the first interior. The normal world including Blue hell is called 'Heaven K'. Because the names and letters are the most frequently used designations, those are used on WikiGTA. The number is the 'official' script-ID, but the name and letter have been given by fans. Every heaven has it's own name, letter and number. In some heavens you'll find one or more yellow doormarkers, which will take you to an interior in another heaven.
Interiors in another heaven are invisible, but it is possible to 'transfer' between heavens using markers. When you enter a interior, every interior in that heaven will be loaded That's why you can see Ammu-Nation through the mirror in the barber. Those groups are called 'Heavens', because they are located high in the sky. It is possible to visit those interiors, but first you should understand how this universe works, due to the complexity of the hidden interiors universe of GTA San Andreas. Among the interiors you'll find some interesting and unused buildings. GTA San Andreas contains more then hidden interiorsspread over 17 'heavens'.
