Mt. Rushmore
Mt. Rushmore is one of the oldest surviving landmarks of human civilization on Earth. On the side of a cliff is carved the faces of four historical figures from over 30,000 years ago, later identified as George Washington, Thomas Jefferson, Theodore Roosevelt and Abraham Lincoln. The local Terrafelis tribes refer to them as the Old Watchers.
Mt. Rushmore Sanctum
The Valerian Empire also visited Mt. Rushmore long after its original construction to continue maintaining it and even building new facilities around it in secret. According to legend, deep inside the cliff face is a hidden chamber that reveals how to open the Olympus Mons Vault, the largest surviving repository of pre-Blackout knowledge. The only way to open it is with neural scans of the four presidents, something thought to be impossible given that all of them are long dead. However, the next closest candidates are virtual life form depictions of those presidents from the Olympus Mons Historical Series. Thus, by theoretically assembling virtual life form copies of Washington, Jefferson, Roosevelt and Lincoln, one could open the hidden chamber.
Most copies of the Olympus Mons Historical Series have long since degraded, leading many to believe that the Mt. Rushmore chamber is impossible to open. However, several presidents derived from backup copies of these lost simulations were sighted throughout the galaxy that could potentially serve as candidates. For example, Abraham Lincoln was spotted on Yeddo working as a Rampant hunter while George Washington was identified as a machine god on Columbia. Theodore Roosevelt is known to be hidden away in the Varlog Sanctum, closely guarded by the Archaetronians where only those that can prove themselves worthy in a brutal combat challenge are allowed. The only known copy of Thomas Jefferson was stolen from a Columbian museum and sold on the black market where it's now thought to be in posession of the Soul Brokers.
Inside the Sanctum is a puzzle: a large orrery depicting the planets of the solar system with a geocentric configuration. Using a series of gears and dials, the planets can be configured to display various astrological signs. One must solve a riddle and arrange the planets in a very specific pattern to reveal an encryption key that will provide the combination to open the Olympus Mons Vault. An attempt to open the Olympus Mons Vault with brute force guessing is estimated to take one trillion years, leaving this puzzle as the only practical option.