One of the commenters on the thread had something interesting to say:

I believe the most mysterious is number nine, the Indus Valley civilization. The main reason why is, because nearly all other civilizations have been thoroughly excavated and unearthed, their mysteries relatively solved and very well portrayed in our modern culture. Most of these are iconic in our collective psyche, but I guarantee that Mohenjo Daro in modern day Pakistan and similar remains in parts of India are hardly known to the general public. Even learned scholars from prestigious universities, such as Harvard and Yale, barely know anything about the fascinating Indus civilization. We know so much about Egypt, Mesopotamia, the Greeks, the New World civilizations, and even a great deal about China. But as I insist, the ancient river valley of the Indus still hides her mysteries, and it is imperative that we try to uncover their secrets, for they left behind some of the best engineering and architectural projects from the ancient world. We’re only now thinking about sustainability and renewable energy, these people from over 3,000 years ago were far more practical than us. They did not burn fossil fuels and bring erratic weather patterns upon themselves, as we are clearly experiencing in the 21st century!

What do you think is the most unusual one on the list?  Many people still think the pyramids at Giza and Egyptian archaeology is the most mysterious but it is still debated.

Feel free to comment with any of your ideas on these!!