Millennia from now, future archaeologists will probably wonder why France decided to build a big metal tower in the center of Paris. They'll come upon the ruins of Las Vegas and puzzle over why 20th century humans built a glitzy metropolis in the middle of nowhere. They'll find the ruins of the Brooklyn Bridge and marvel over how it could have been constructed using only steam-powered machines.
For modern archaeologists, the ancient world continues to hold many secrets. Civilizations disappear, and thousands of years later, researchers are left to pick through the ivy-covered ruins of their cities.
The following is a list of five awe-inspiring sights whose codes haven't been cracked yet. Who built them? Why did they build them? Did humans even build them at all?