The Skydoor consists of three yellow floating ENEX markers. These yellow markers look like arrows, and they have a slight bouncing animation. They are normally used to teleport a player into an interior when they walk into one.
Two of these ENEX markers can be found floating in mid-air above Fort Carson, and one above the Johnson House, only a few feet away, but they cannot be accessed in a normal game because they are higher than the game's height limit. Also, in order for the player to be able to enter a marker, they must be standing on a solid surface. Therefore, the Skydoor cannot be reached with airplanes, helicopters, or a Jetpack. The ENEX markers are located at the same height but placed a few meters away from each other.
If entered, both ENEX markers will transport the player to the interiors of unused beta Burglary houses. Inside the houses, the player is able to pick up random appliances and electronics, such as televisions, microwaves, and radios, much like they can during the Burglary side-missions.
Inside one of the interiors, there is a pedestrian who will always spawn inside one of the rooms. They are always hostile to the player, and if the player kills them, their body will not produce any blood. This pedestrian sparked a small myth in itself when players realized that they never bleed after being killed. However, it is likely that this is simply a minor graphics glitch, as the pedestrian behaves the same way any other one would.
Inside one of the interiors, there is a set spawn point for the Skateboard. This is the only skateboard spawn point in the entire state. However, the player must use mods in order for the skateboard to spawn in the first place. If the player finds the skateboard and picks it up, they will pick it up like a weapon, and will wield it similarly to a knife.
The exact locations of the Skydoors are very high in the sky, above the corner of a fenced field across the road from the Cluckin' Bell in Fort Carson and the last one above The Johnson House.
The Skydoors in Bone County are believed to be connected to the Epsilon Program. A popular fan theory on joining the cult mentions the "Puerto Amarilla Valley". This, with a few grammar errors, translates from Spanish as the "Yellow Door Valley." As the Skydoor is both yellow and a door, fans have claimed it is one of the locations that the player can find the Cult, or even their deity, Kifflom.
Rockstar has stated that the markers lead into unused interiors. The interiors were originally going to be houses that the player would use in the Burglary side-missions, but the interiors are never used in the final version of the game. Rockstar said that they put the yellow markers very high in the sky, where they knew nobody would think to look for them.
Unfortunately, the Skydoor cannot be entered without the use of mods or hacks. The Skydoor is floating in mid-air a few feet higher than the height limit for airplanes and helicopters, so it cannot be accessed on a normal game. It is possible to see the Skydoor during the mission Stowaway, but because the player is using a parachute, it is impossible to access. There are jetpack mods available which the player can use to "fly" above the game's height limit and enter the Skydoor.