Kryptos
This is significantly less creepy than the other things here, but I’m going to go with the Kryptos sculpture at the CIA headquarters.
The sculpture was created in 1990 by Jim Sanborn and consists of four parts. The first three parts were decrypted in 1993, but the fourth part remains a mystery 25 years later. It’s gotten worldwide attention from cryptoanalysts, but it still hasn’t been solved.
The odd thing is that Sanborn doesn’t have much of a background in codebreaking or cryptoanalytics, yet he still created a code that has stumped the best experts in the world.
Story Credit: Reddit/crono09