Blue hell is the name given in the player community to the area underneath the landmass in all Grand Theft Auto games. The name first started in Grand Theft Auto III in which the area underneath the landmass is usually blue, but its color usually depends on what time of the day it is.
The player is not supposed to be able to access blue hell, but several game glitches, including holes in the world terrain, can result in the player ending up there. When the player falls into blue hell, they will fall through the air for a few seconds before re-spawning on the nearest pedestrian pathway (or vehicular if the player is in a vehicle). This is a game mechanic implemented so the player doesn't end up falling indefinitely if they ever accidentally enter blue hell. Blue hell in Grand Theft Auto: San Andreas is the easiest to access and explore with the use of the in-game Jetpack.
In the handling.cfg file, if the "float percentage (%)" value of a vehicle is set to any negative number, it will automatically go to the blue hell when it is driven into the water.
Removal of map objects and/or collision data allows the player to create "holes" that can be used as entrances to blue hell, although doing this is not recommended.
Using the car spawn mod can lead to entrances into blue hell. Since the user can opt to have Niko "warped" into a car upon being spawned, this allows the player to stand to the left of a non-enter able building and spawn a car, which will result in the car being spawned inside the building and blue hell, with the player inside. This mod requires downloading a "script hook," which will disable any online capabilities of your game.