My breakdown of the mural...
1. On top there is an eye, or UFO, that appears to be shining. (Our UFO)
2. It fits the face of the Chilliad perfectly from one side, and decently from the other (Could mean symmetry?)
3. There are lighting bolts or power strikes on the outside(Thunder)
4. There is clearly a Cartoony UFO in the 1st square, notice it is much rounder than any UFO we've found ingame
5. There is a cracked egg in the middle square, maybe hinting at the fact this is just Easter Egg?
6. Clearly a man with a jetpack, notice the handles he is holding on to
7. There are only 3 'Connections' inside the mural that make it to the outside, one straight from the Jetpack, one on the top, and one directly above the UFO box, could indicate hidden tunnels
8. There are 'X's in boxes on the Mural, but some are washed away and look a bit more like just dashes
9. There are some sort of 'Stretch marks' leading down from the mural, this i have no explanation of
Some theories based on above...
2. It fits the face of the Chilliad almost perfectly, so for me, it must be the overlay over the map.
7. Maybe, if we had a 3D model of the mountain, we could pinpoint exactly where the tunnels lead out to, maybe revealing something.
We also know there is some 'Non solid' bits of the mountain, that somebody has managed to throw bombs into, and they don't fall into Blue Hell, if they did, we wouldn't hear the bang as it being close, so there is something solid in the inside of that no solid wall.