Inside the Mt. Chiliad lift station is a mural that pictures what looks like a line drawing of Mt. Chiliad. The image depicts an array of connecting lines that create "paths" to five boxes that contain red markings that appear to be X's. At the very top an eye or UFO can be seen.
On the bottom are three larger boxes. Starting from the left, one of what looks like a UFO, one with a cracked (or cracking) egg, and another that looks like a person ascending with a jetpack
Since its discovery, many theories about how to correctly interpret the mural have emerged. Common speculations include theories that the mural's lines and boxes depict actual tunnels running through Mt. Chiliad that physically lead to rewards.
Some believe the diagram relates to a more metaphoric path of in-game key decisions / events. Others have tried various methods of overlaying the mural on top of in-game screenshots or the GTA V world map itself to see if anything interesting could be found.
The exact meaning of the mural is still unknown. It's assumed that a jetpack will be the reward for solving the mystery, and possibly a pilotable flying saucer.
Given that there are 5 glyphs on the side of the mountain, it's possible that the five boxes correspond to the glyphs.
Four of the five glyphs on the mountain matched up to an overlay (seen below) but the fifth glyph was misplaced, meaning the overlay was wrong.
Map Overlay - Glyph LocationsEdit
One of theory originally posted on GTAForums.com used the mural as an overlay showing glyph locations lining up the eye at the top of the mountain (red dot) with the glyphs on the the side of the mountain (green dots) leaving one half faded glyph (green circle). As mentioned in the section above, this overlay is not perfect, but it is the only method of the overlay technique that has yeilded promising results.
The blue circles simply represent the final 3 boxes on "The Mural" and as far as we know this overlay doesn't point out where those things actually are.