While many game franchises are known for masterfully hiding easter eggs, few are quite as famous for their easter eggs as the GTA franchise. While the hidden secrets have only become a staple of Rockstar's hit open world crime franchise in the 3D era, this trend continued on into the HD era. One of the most famous easter eggs of all time is probably the creepy, beating heart chained to the inside of the Statue of Liberty knockoff found in GTA IV.

However, eventually all games have their secrets laid bare, either due to blind luck or thanks to data-miners digging out any easter eggs from the code rather than finding them the hard way (of course, this only applies in cases where digging through the code isn't the only way to find said egg).
All games save for GTA 5, it seems. Sure, plenty of easter eggs have already been found, including several UFOs, the frozen alien in North Yankton and most recently the Bigfoot Peyote. However the biggest and possibly oldest mystery of the most recent installment of the franchise hasn't been solved yet — maybe because it couldn't be?
Quick history lesson: The tallest peak in GTA 5's version of San Andreas is Mount Chiliad. This was also the case in the game San Andreas, though the location of the mountain, as well as the peak itself, were different. A cable-car is the easiest way to access the top of the mountain. Hidden under the cable-car station is a mysterious mural, which seemingly depicts a cross section of the mountain with several odd symbols etched into it.
There is also writing on the wall saying "come back when your story is complete". If the player visits the peak with 100% completion during certain weather conditions, a UFO is seen hovering above the mountain.