21. The Pyramids of Giza
The pyramids of Giza are some of the most famous ancient architectural marvels known to man. Considering that over 2 million limestone and granite blocks were used to build the main pyramid, it’s easy to see why they’re known the world over.
But despite being so popular, there is a deep mystery around the construction techniques that were used to build structures of such a massive size, and with such precision.
To further add to this enigma – the pyramids also contain a secret network of inner chambers and passages that run hundreds of feet long. Historians are still unsure of the purpose these passages served.