Answer

There are many but my favorite ancient mystery is that of the ancient Incan society of Machu Picchu. It's amazing. There are hundreds of walls that are made up of multiple rocks and some boulders that are up to 40 tons in weight. The rocks and boulders are cut and fitted together so perfectly, a piece of paper can't fit In between them. The boulders are massive but they have no remnants of the wheel in Machu Picchu! How did they build this? How did they move them? There is no plaster used. These are real rocks cut incredibly perfect to fit with others. I have a lot of pictures. No one knows how they could have made something so complex and moved such massive boulders and cut them into 90 degree angles.

Look at the bottom 3 boulders. They are cut precisely to adhere to the boulders above.

Same here

This picture is unusual as well. Those 2 compartments are nearly identical and that center stone is made to fit perfectly to the top stones and bottom stones to be flush. Just incredible. Look closely at the bottom right boulder. How would someone make that with the technology they had? No one knows

And another one. The boulders on the right and left are different sizes but the center rocks are the ideal angles on both sides to fit in the middle so they are flush.

On the corner, the walls all have the same curve to them

Again, walls constructed of rocks are fit together with immense precision.

This is their beautiful city. A masterpiece shrouded in mystery.