Nephilim. The Old Ones
Cover-up
In 1982 I subscribed to a magazine called Science 80. One of the best of their publications was about the ancient ones and their mechanism to predict the Spring and Fall Equinox and the Winter and Summer Solstice to the day, with shafts of light between stone slabs.
The magazine later became involved with leftist and loonie crap and it died. Long before it did, I stopped my subscription.
They were not aliens, but people who lived long ago were smart. They survived. They knew Nature.
Nah. I've been to Chaco Canyon. Amazing place. But if you are over 5' tall, you are bending over most of the time wandering through the structures. I'm 6'-1" and it was a struggle getting through the place.