This episode suggests that various sites around the world; such as Gobekli Tepe in Turkey; Inca ruins of Sacsayhuaman in Peru; the Carnac stones of France; and Zorats Karer in Armenia, show construction techniques and mathematical concepts that were not believed to have been known at the time.
Carnac Stones, France
More than 3,000 prehistoric standing stones were erected at some stage during the Neolithic period, probably around 3300 BC, but some may date to as old as 4500 BC.
Local tradition claims that the reason they stand in such perfectly straight lines is that they were a Roman legion turned to stone.
Zorats Karer (Karahunj), Armenia
This site with about 223 large stone tombs was explored by a team of archaeologists from University of Munich which they concluded it was mainly a necropolis from the Middle Bronze Age to the Iron Age and added it may have served "as a place of refuge in times of war", possibly in the Hellenistic-Roman, 300 BC - 300 AD.About 84 of the stones feature a circular hole, although only about 50 of the stones survive. They have been of interest to Russian and Armenian archaeoastronomists who have suggested that the standing stones could have been used for astronomical observation.
Stonehenge was constructed in three phases and has been estimated that the three phases of the construction required more than thirty million hours of labour. Speculation on the reason it was built range from human sacrifice to astronomy.
Here is the episode about these ancient structures if you are interested. There are much more of them on Earth and more waiting to be discovered.
You can find all episodes of the program on YouTube, presenting theories of ancient astronauts and proposes that historical texts, archaeology and legends contain evidence of past human-extraterrestrial contact.