There are structures in bout a dozen places all around the world built using 50-200+ tons stones...which there is not a crane on planet Earth that could place them where they sit today, much less a machine to cut and plane their edges perfectly straight and flat...nor transport them to the sites. Many of which fit the adjoining stones so tightly...without mortar...that you couldnt slide a piece of paper between the 2 impossibly big stones.
We (humanity) have no idea how these structures were built thousands of years ago...especially since many of them cannot be replicated today with every imaginable tool at our disposal.
There are a few batsheit crazy archeologists that try and say hilariously that the huge obelisks in Egypt that get up to about 20 tons and maybe 60ft tall were somehow cut from the bedrock using wood and stone tools (no way in Hell, regardless of the timeframe) but NOBODY is brave/stupid/delusional enough to put forth a method by which people thousands of years ago with zero power tools, machinery, etc magically somehow cut and placed... often perfectly square, straight, flush, level, and plumb...stones that weigh in the hundreds of tons. In some cases geologists theorize these stones even came from hundreds of miles away. In others like Machu Pichu, they sit atop mountains that are only traversable on foot.
I am not saying aliens built them, i have no idea. Humans from this iteration of life on planet Earth damn sure didnt build them though...since we cannot even build them now. I think as ground penetrating radar and other technologies that can peel back these layers of dirt progress we may find out there were once civilizations far more advanced than ours on this planet. The further back in time things happened, the deeper underground they are...so we have no idea. Or it really could be aliens. It is quite suspicious that at similar times in the past, pyramids were built both in south america and africa...2 civilizations that from what we can tell never, ever interacted with eachother. Both civilizations built their structures and monuments to correspond to stars and planets in the sky as well...definitely some crazy stuff we still do not understand. The pyramids were childs play compared to some of these other sites though...as far as the size and mass of the often perfectly cut stones used to construct them. Hopefully as our technology improves we will have a better understanding of how and why these megalithic structures were built.