Blue Hell

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 from GTA III in which the area underneath the landmass is usually blue.

The player is not supposed 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 through Blue Hell, they will fall for a few seconds before respawning on the nearest pedestrian pathway (or vehicular if the player is in a vehicle). The Grand Theft Auto: San Andreas Blue Hell is the easiest to access and explore with the use of the in-game Jetpack.

Grand Theft Auto III

 * Going eastwards up the sloped alleyway in Saint Mark's, north of Marco's Bistro on Portland. The wall at the end of the backyards on the left hand side is not solid and you can easily fall through. After a few seconds in Blue Hell, the player will land in the middle of the main road just to the east of where you were.
 * In the Staunton Island LCPD Station, one of the higher walls is not solid. The player can easily enter and fall into Blue Hell. After the fall, the player will land in front of the station's car park entrance.
 * If the player rides on the subway and enters one of the trains at the second door of the first car, and then exits the train at Francis International Airport, the player may appear above the station. Walking above the tracks (the player must jump over one of the first electricity things in the tunnel´s ceiling, otherwise the player will fall of the roof) and off them will make the player fall into Blue Hell and reappear at Francis International Airport. This is a good method to reach the whole Shoreside Vale without cheats or mods, if you not have unlocked it (if the player die or get busted there, the player will respawn on the hospital/police station on the nearest island that the player have unlocked). Alternatively, the player may decide to walk across the tunnel (tested on PC and iPad version).
 * On the Newport/Fort Staunton Expressway under the Callahan Bridge on Staunton Island, there is a corner that is unsolid. If the player goes to it, the player will fall into Blue Hell and reappear on the sidewalk under Callahan Bridge.
 * On Staunton Island, some sections of the Liberty Memorial Coliseum are unsolid, and if the player manages to get into a hole, the player will fall into Blue Hell and respawn on the nearest street.
 * At Joey's Garage on Portland, if the player does the "Accessible Joey's Garage-glitch", one of the walls isn't solid. If the player goes through it, the player will fall into Blue Hell and respawn on the street.
 * At Ghost Town, there's a large Blue Hell entrance where the half of the town is situated.

Grand Theft Auto: Vice City
If, when playing the PS2 version, your PS2 is unable to load or have problems loading, the ground might not be loaded and you will fall into Blue Hell and spawn on land a few seconds later. Alternatively, the ground may become untextured and the game will freeze.

If you use the Rhino's turret combined with the flying cars cheat and you somehow reach the top of buildings past the game's height limit, you may see a large hole instead of a roof. This acts as an entrance to Blue Hell.

Grand Theft Auto: San Andreas

 * In Esplanade North, San Fierro, the player will see several docks jutting out into the San Fierro Bay with warehouses situated upon them. Next to one of these docks is a submarine called the USS Numnutz. If the player swims directly down from the submarine to the bottom of the waterway next to the dock, they will notice a greyish color which does not match the color or texture of the surrounding rock. This area is actually a hole that the player can swim through to enter Blue Hell.
 * If the player stands on top of a San Fierro tram and takes a picture with the camera while on the tram, CJ may fall into Blue Hell and after a few seconds, and eventually land on the sidewalk.
 * Also in San Fierro, there is a large clothing store in Doherty, opposite the tract of wasteland. The top portion of the wall (between the top floor and roof) is not solid, and can be accessed with a jetpack.
 * In Mulholland, Los Santos, there is a newspaper shop with a non-solid areas in the roof which will lead to the interior of the building, and from there to Blue Hell itself.
 * Get a jetpack, then go to the Mulholland Safehouse. On the empty pool, one of the corner is not solid. Walking through it will make you fall underneath Los Santos. You can use the jetpack to explore the city via the "underground". Otherwise, if you walk through the unsolid wall without a jetpack, you'll fall for a while and respawn in the street.
 * At Las Venturas Airport, one of the walls of the main building has a glitch wherein running at it enough will pop the player through into Blue Hell. The wall is located where the fence around the runway meets the building.
 * Also in Las Venturas, there is an unmarked Pay 'n' Spray east of the casino in the Come-A-Lot district. The southwest corner of the Pay 'n' Spray is not solid; if the player enters that spot via jetpack and lets Carl Johnson sink into the hole for awhile, he will fall into Blue Hell, later automatically spawning on the sidewalk outside the Pay 'n' Spray.
 * Outside the Ammu-Nation in Red County, enter the Jetpack cheat (or use your own) and walk in front of the entrance. The yellow marker will disappear. When this happens, lift up and you will fly into Blue Hell.
 * The player can use Blue Hell to re-enter the Area 69 interior after the Black Project mission.
 * Sometimes after the mission End of the Line, the interior of Big Smoke's Crack Palace will disappear, however the wall will not repair itself. This creates a large hole to Blue Hell.
 * If you fly far out enough into the ocean and swim to the bottom of it (you must have full health/lung capacity so you live long enough) the ocean floor is not solid, and you will respawn somewhere in San Andreas. If you die in the process, you will spawn dead somewhere in San Andreas on the sidewalk.
 * There is a building in Juniper Hill, San Fierro, in which one spot on a wall is not solid, but can only be accessed with a jetpack. It leads to the interior of the building, and then to Blue Hell. The building is between a clothing store and a safe house.
 * Also in Dillimore, Red County, there is a Discount Furniture store. It's name is not shown on the shop, but it is a building with a lot of signs and stickers on the windows. It's not an enterable building, so there will be no yellow marker at the entrance, but go to the entrance with a jetpack, and fly up, and the roof will not be solid. Then you just have to try to get in.
 * If you spawn a vehicle with the driver's door in a wall (so that the vehicle is partially immersed in it), get in it, and then exit it, CJ may exit on the driver's side. Since the door was inside the wall, CJ will now be inside of it as well and will fall to Blue Hell before respawning on the nearest street.
 * In Tierra Robada, below the Robada Intersection and between the Garver Bridge and the Kincaid Bridge, there is a section of unsolid terrain along the small cliffside above the sea. Obtain a Jetpack and fly downwards against the section for a while, and pretty soon,CJ will be popped into Blue Hell.
 * Before the mission End of the Line you can blow up the wall with explosives and inside will be Blue Hell.

Grand Theft Auto: Liberty City Stories

 * Near the hospital in Staunton Island is a small construction site where the player can find a ramp and jump to a platform. The player can go inside the glass of the windows in a corner and will fall into a tunnel (the same tunnel seen during the mission "Bringing the House Down"). During the mission "Bringing the House Down", the blue hell under Fort Staunton is solid so the player wont fall through (Tested with the use of Cheat Device). See Carson General Hospital for a full description.
 * (Note: You need to activate the 'Perfect Traction' cheat to perform this glitch). In front of Sweeney General Hospital at Portland View there is a place where there is an Angel and an Esperanto. Get a car, jump using 'Down', drive it to the taller roof, get out of the car or drive it and jump to the white lanes. You will fall into the Blue Hell and will respawn at the nearest point at Portland View or Chinatown.
 * (Note: Activate the 'Cars in Water' and 'Perfect Traction' cheats to perform this glitch). Go to the Shoreside Lift Bridge. Drive straight while the bridge is still lifted up. Go to the Shoreside Vale end. Jump and drive under the bridge (before a pillar) and when you have jumped in the bridge, go to the grass. You will fall into the Blue Hell and respawn later at the Francis International Airport end of the Porter Tunnel.
 * On Staunton Island, some sections of the Liberty Memorial Coliseum are unsolid, and if the player manages to get into a hole, the player will fall into Blue Hell and respawn on the nearest street.

Grand Theft Auto: Vice City Stories
Go to the side of the Prawn Island bridge going to Vice City Mainland. Run onto the wall until you fall in Blue Hell, and you will respawn at Diaz's mansion.

The player can also enter Blue Hell by running towards a corner and a tank hits the player. This with cause the player to fall into Blue Hell and spawn at the nearest land 10 to 15 seconds later.

Note:Plan very carefully before doing this! Use a vehicle and go to an invisible wall near the freighter in Viceport. Plan carefully, as this can result in many failures. Drive and bail out and swim at the invisible wall until you entered the wall. Swim until you seen a hole, then use the hole. You will fall in the Blue Hell and reappear in the nearest land.

Fly a Skimmer to the Prawn Island bridge (either InterGlobal Films or one of the mansions). Exit the Skimmer and walk until you are swimming. In the InterGlobal Films loop, swim all the way. If you use the mansion, this is dangerous. It will be split into phases. Try to swim carefully as you can suddenly reappear into the surface. You will gone down and swim more. The end of the route is at the road lanes between the three mansions and InterGlobal Films road. You will fall and spawn later. If you fall on the box, you will spawn in the Mendez Mansion. Note:The place is at Downtown-Prawn Island bridge.

Grand Theft Auto IV

 * Sometimes, when the player utilizes the Swingset Glitch with the Tudor swingset and a large truck like a Benson or Yankee they may be thrown into Blue Hell for a few seconds and then back on the surface somewere near the swing.
 * Part of the south side of the Rotterdam Tower (just beneath the first high ledge) is non-solid, and can be entered with a helicopter to access Blue Hell. From here, the player can enter interiors they could not enter before, go underwater, and explore some buildings. The player may accidentally re-enter the "real world", however, as some "real" interiors are not visible from Blue Hell (this is especially true of tunnels), and entering these will restore the player to the normal game world.
 * Upon entering the Schottler Medical Centre, turn right, and then right again. You will be in a long hallway, with some windows and hospital privacy screen directly in front of you. Throw a grenade near the wall; this will shatter the windows and move the screens. You can now jump through the window and walk around in Blue Hell with some of the hospital's decals, doors, even pedestrians invisible for a while, however, if you venture to far towards the street, the surroundings will reappear. A video of this can be found here; http://www.youtube.com/watch?v=_ogSKeOmu6Y
 * Driving east into the Booth Tunnel at high speed may put the player into Blue Hell for a few seconds.
 * Getting hit by a train underground while in a car may put the player into Blue Hell for a few seconds.
 * At Pier 45, facing the back from the water, swim to the left were there is a bunch of wooden posts. Climb on to one. Niko will jump over it as if it were a fence and land in "water". Swim around and you will fall into Blue Hell.
 * Sometimes if the player is in a destroyed helicopter (i.e no rotors/tail) and the use the health and weapons cheat to repair the helicopter, the player will fall into Blue Hell, and can explore it with the helicopter. The player can return to the game world by flying up through the ground.
 * Near the West River helipad, there is a cement wall partially covered in rocks. If you swim against this in the correct spot (The rocks should be just below the waterline) for a few seconds, you will fall onto the seafloor. You can walk through the cement wall into Blue Hell, although it is very easy to fall into the nearby Booth Tunnel and return to the real world.
 * Note: A helicopter is needed for this glitch! In Dukes, near Crockett Avenue and the big rollercoster, there is a Go-Kart racing track. If the player crashes the helicopter with the nose to the ground on the south part of the track, he should sink into Blue Hell. If the player exits the helicopter, they will spawn on whichever street is nearest, or in the water.
 * At the second building by the Leapers Bridge between Albany Ave and Hematite St., go to the front entrance of the building and then jump into the handrail where the matress is. You will fall into Blue Hell for 5 seconds and end right back out into Hematite st.
 * In Middle Park West in Jade St., there's an alleyway, which if you get stuck in its entrance (at least with the FIB Buffalo), with a side up and the other on the floor, get out, and if you then try to get in again you'll fall into      Blue Hell.
 * Sometimes, if you spawn a vehicle in the subway it will seem to not be there, but if you press F/Y/Triangle, there is a high chance the car will be right under the subway line, causing the player to fall in the car into Blue Hell (tested in PC version).
 * In the dockyard near Francis International Airport, there is an entrance north of the big oil tanks in between the pipe lines and the ground. (Only works in helicopter)
 * Go all the way south of Star Junction. You will see a small place with a few trees and benches. Where the building is on the right, you'll see a space. Walk into the space and you'll fall in Blue Hell landing somewhere on the road.
 * There is a large fountain-statue located, near the hospital in Easton. Take an Annihilator (It is the easiest to do so), make a little distance to the statue, and ram its nose (the Annihilator) into the left side of the statue. You may have to redo it sometimes, and you may also have to use cheats. Tested on PlayStation 3.

The Lost and Damned

 * In the mission Buyer's Market, whilst getting to the ground floor, if the player aims an explosive correctly, he can launch himself out of the building into Blue Hell.

The Ballad of Gay Tony

 * In the mission Boulevard Baby, after the shootout in the Bahama Mamas club, Luis can fall through Blue Hell when walking up the stairs after the shootout, then respawn outside of the club.

Via File Hacking/Modding

 * In the handling.cfg file, if the "float percentage (%)" value of a vehicle is set to a negative number, it will automatically go to the Blue Hell when 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.