Can you tell me about some of the ancient structures that people think were built by aliens? Include at least 3 different structures. Give it to me in a bulleted list with sentences with an introduction before the list. Be sure to mention info about the civilization that built the structures and when it was likely built.
- The pyramids of Egypt are gargantuan monuments to the ancient pharaohs built over 4,500 years ago. The sheer size of this construction project has led many to speculate that they must have had some extraterrestrial help. The pyramids are made up of millions of stones that each weigh at least 4000 pounds. 
- Sacsayhuamán is a massive fortress built by the Incan Emperor Pachacuti in the Peruvian Andes, constructed with precisely chiseled stones put together in a perfect arrangement like a puzzle. This structure was built around 1000 years ago and the stones were brought more than 20 miles before being used. 
- Teotihuacán, or the City of the Gods, is an ancient city that could house more than 100,000 people and sat near modern-day Mexico City built more than 2000 years ago. For many years, scientists believed the Toltec culture built this sprawling city that had multiple alignments with different astrological events. But it's now believed that this city predated that culture by a few hundred years and the people who built Teotihuacán remain a mystery. 