Hidden interiors universe
The hidden interiors universe, also known as the black hell, consists of all the interiors of enterable buildings of a GTA. These locations may be accesable through a doormarker ('enex') like the interior of the Washington Mall or Burger Shot, but a lot of interiors are only used in certain missions or cutscenes, like the Pleasure Domes or the Greasy Chopper. Some interiors even aren't used at all, like the Warehouses. These interiors can be reached by travelling through the hidden interiors universe (by exploiting a bug or using a trainer). The interiors universe should not be confused with blue hell, although there are some similarities.
How does it work?
To understand and explore the hidden interiors universe, you should first understand how the interiors of GTA work. First of all, interiors aren't nessecarely located in their exteriors. For example, the interior of the Ganton Gym is located somewhere in Fern Ridge, high in the sky. When you enter a building through an enex, the game teleports you to the interior without you noticing this. The radar goes blank, and your position on the ingame map won't change, so there is no way of telling your real location without using something like a trainer.
Still, if you would go to fern ridge, use an unlimited-height-cheat and flew to the location of the Ganton Gym-interior, you wouldn't see it. This is because the interior hasn't loaded if you haven't entered the correct enex. The interiors only visible in their own 'world'. Those 'interior worlds' are called 'heavens'. GTA San Andreas has no less than seventien heavens. Each heaven contains several interiors, which are all loaded when one of them is visited.
Imagine you could leave an interior, without leaving through the door, thus keeping the interiors loaded. This means you could travel to another interior of the heaven you're in and visit interiors you're not supposed to reach. Lets illustrate this to make things easier to understand. The right picture depicts three houses. They all have an interior. The red house is the Ganton Gym and the yellow house is the Crack House (From the mission 'Cleaning the Hood'). You would like to visit the Crack House again, but as you know, there's no enex outside.
Now look at the left picture. This is the heaven of the Ganton Gym. As you can see, the Ganton Gym interior is loaded, and so is the Crack House because it's in the same heaven (The blue interior is pictured in gray because it isn't loaded, which means it's in another heaven). The only thing you have to do is leave the Ganton Gym interior without going through the door. A bug makes this very easy: The roof above the door isn't solid. Spawn a jetpack with a cheat and fly through the roof. You'll enter a black void: The hidden interiors universe, heaven "A". Now all you have to do is flying to the other interior.
Before you try to fly to the Crack House, there are a few things you should know. First, you can't use the jetpack without killing everybody in the Ganton Gym. The interiors are located very high in the sky. By killing everybody in the gym, you can use the jetpack at this height. Nobody seems to know why, though.
Second, you can't fly above water at this height: You'll be dragged down. Water in the interiors universe is known as 'the red zone'.
And lastly, remember your not at the location shown on the map! You have been teleported to another location without seeing this on the map. The map will continu to use this incorrect location when you leave the interior to explore the interiors universe.
GTA III does not have a real interior-heaven. There are some unreachable interiors like Ghost Town and Joeys Garage, but those are located in the normal game world or in the blue hell, rather than in an interiors universe.
GTA Vice City
GTA Vice City uses the first interior-heavens. Interiors like the Vercetti Estate aren't located in the normal game world. Although the interiors are only visible in their own heaven, most of them are located in their exterior. Exploration of the heavens is not easy, as most heavens aren't accesable through normal interiors.
GTA San Andreas
|Main article: Hidden interiors (GTA San Andreas) and Hidden interiors map (GTA San Andreas)|
GTA San Andreas contains more than 140 interiors. Most of them are located in the sky. The jetpack makes it very easy to explore the interiors universe. Due to the easy acces and the interesting interiors, this interiors universe is by far the most populair of GTA.