The Mount Chiliad Mystery is a myth and an Easter egg in Grand Theft Auto V that refers to a series of mysterious markings scattered around Mount Chiliad, the largest mountain in the State of San Andreas.
Inside the Cable Car Station at the summit of the mountain, there is a large map painted on a wall that appears to be a map of Mount Chiliad. Scattered across the map are various odd symbols, seemingly pointing out the locations of secret objects on Mount Chiliad.
A strange symbol can be seen drawn above Mount Chiliad on the map. The symbol appears to be an eye. This symbol is called the Eye of Providence, also known as the All Seeing Eye, which could be a reference to the Illuminati. The symbol could also be the UFO, either the sunken wreck or the one above Mount Chiliad itself. The symbol is similar to the symbols that give clues as to when the character can find the UFO. In this case, at the point where the UFO, just above the observation station, lined up with the cable car, the map may be accurate to Mount Chiliad. (Something what appears to be a tunnel to the surface on the map, might have something to do with the mystery.)
Another strange symbol can be found marked in red underneath the viewing platform. This is believed to be related to the achievement From Beyond the Stars, where some UFO parts must be collected, as it shows what appears to be the map of GTA V with some markers on it. The last drawing is etched behind the viewing platform that reads Come back when your story is complete. If the player returns to Mount Chiliad after getting 100% Completion, a UFO will appear floating in mid-air above Mount Chiliad. This has been proven to be an Easter egg.
These "Easter eggs" may also be a reference to the Illuminati, an old secret society. In the Illuminati, there is a symbol called the All Seeing Eye, which is an eyeball placed onto a pyramid. This may be what the drawing inside the cable car might be, but Mount Chiliad is the pyramid. The strange red mark also bears a resemblance to an eye, and Come back when your story is complete could be a reference to the New World Order, the Illuminati's scheme to take over the world and make 1 totalitarian government.
Solving the Mystery
The mystery behind UFOs, the Underwater Hatch and the possibility of a Jetpack in the game may be on the verge of being solved. Players have partially overlaid the vertical cutaway map of the mountain displayed on the wall of the Cable Car Station on top of the Northern reaches of the in-game map and certain areas of interest seem to line-up with markings on the diagram on the wall.
If overlaid on the map directly North-to-South, players have found that the UFO-like marking that adorns the top of the diagram overlays directly where the Sunken UFO Wreckage is located.
Unfortunately, this is where the common sense ends and community consensus is torn, as the overlay now must be flipped horizontally and seriously up-scaled to the point, where the second UFO-like marking in the bottom Left corner of the diagram would mark a location far off the Eastern side of the map. However, if this is done, the Jetpack marking on the diagram would overlay Fort Zancudo, a heavily guarded military installation. Players have theorized that this would be the most likely place a Jetpack would be discovered, as reflected in San Andreas, where the Jetpack could be found inside Area 69. The drawing likely indicates that a UFO could appear given some additional work after all 50 UFO parts (Spaceship Parts) are collected, after the UFO's appearance presumably the jetpack would be unlocked.
Some players have reported finding a Jetpack in Grapeseed County. The diagram could theoretically present a similar location, by placing the UFO-like marking that adorns the top of the diagram at the Paleto Bay area, then, by following the highway from Paleto Bay through Mount Chiliad and Mount Gordo to Grapeseed, a resemblance can be made between the line on the left side of the pyramid and said highway.
It is possible that the jet pack alluded to the Thruster from The Doomsday Heist. However, this may not be the case, since the update for The Doomsday Heist was released four years after Grand Theft Auto 5 was initial release.
The Easter Egg
The Easter egg marking on the diagram now marks the spot off the Eastern shoreline, where the Underwater Hatch Easter egg can be found. Players have theorized that the egg may refer to the Epsilon Cult's beliefs, in which it is stated that the world hatched from an egg.
The Peak of Mount Chilliad
All three locations form a near perfect triangle if they are connected between each other using lines covering the shortest possible distance. Interestingly, if altitudes originating from each interior vertex are drawn, the other center of the triangle overlays the Peak of Mount Chiliad exactly.
The X markings
Players have theorized that the X markings on the diagram of the wall of the Cable Car Station are not to be considered as haphazard co-ordinates, but rather an indication of the number of strange markings within the triangle, as no matter how the diagram is skewed or scaled when overlaid the game map, none of the X markings line up with any of the discovered locations of interest. As only 3 strange markings are believed to have been discovered within the triangle, 2 more are thought to remain, which may reveal more clues to the mystery. It is likely that the X markings are actually the Roman Numeral 10, as 5 of them indicate there are 50 UFO parts, and given the Illuminati theme, the occult does have a fetish for numbers.
The 100% Theory
Some believe that the diagram on the wall of the Cable Car Station simply represents prerequisites required to achieve 100% completion of the game, which is necessary before a UFO appears above the lookout at the peak of Mount Chilliad, which could be represented on the diagram by the UFO-like object that adorns the top of the diagram. Players believe the lightning markings around the diagram could represent the in-climate weather that must be present in order for the UFO to reveal itself.
Some players believe that no horizontal flipping and little up-scaling is required, as the Easter Egg may be found somewhere in the Alamo Sea and the second UFO-like marking in the bottom Left corner may mark a location somewhere over the Chiliad Wilderness. Some believe the map is tied to Mount Chiliad in GTA San Andreas, as there is a tunnel in that version.