5 Mysterious Monuments from Around the World

There are still lots of architectural mysteries out there. See more pictures of famous landmarks.

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?