I Will be updating this article when i discover more information.
All that is said here i have Discovered by my own means.
What is the Mystery Around Mount Chiliad in GTA V.
There are 8 Boxes on the "Map" 5 boxes have a "X" marked. Could the "X" marked boxes refer to the 5 white hieroglyphics found on the Mountain side? they all feature the same Symbol of the "Sun" on the top of the Unknown "Map" and then a Unique symbol just below it.
- Click on an image to Enlarge it.
The other 3 boxes appear to show a U.F.O an EGG and a what appears to be a person flying a jetpack.
The U.F.O could be a reference to the U.F.O Found in the bottom of the ocean just North of Mount Chiliad. You will need the Submarine to dive there.
There is also an Achievement/Trophy "Collect and return all Spaceship Parts" I do not believe this is apart of the Mount Chiliad Mystery due to upon completion you receive a Unique car called the "Space Docker" its a modified version of the "DF Dune Buggy" which features Rope blue lighting around the vehicle, Green headlights and a Futuristic Engine.