With the uniform clean cut and depth, some suggest a laser type mill machine. Very high tech, with the interlocking and machined fitting blocks. Maybe the first Lego Land Park? Diamonds are the third hardest hardest among the earth elements.
Extremes in weather, earthquakes (had one last night) and time, has a lot to do with removing evidence of what past tools and methods of building by humans was used. The earth has always been under constant change.
Aliens,we don't know. But quite a few ancient civilizations, from all points of the world, seem to have a somewhat common links. Myth's like Gods from the sky or across the great waters (oceans),etc giving knowledge to farming, culture, building,laws, religion, writing abilities, etc. Even Gods living underwater. Some "Gods" been said to have white skin, blond or white hair and very tall. Who knows, really.
Take a human from 10,000 years ago and put him in a modern setting. Not much difference from current man in learning abilities, appearance and other skills.