Advanced Ancient Civilizations

Most of the recorded History of the World was written post 2300 B.C., unless we look at what was engraved on tablets or ancient constructions.
Two of the most regarded and recognized scripts used as records for the Ancient World are the Cuneiform and Glyphs or Hieroglyphs. It is very difficult to date such carvings whereas on tablets of constructions.

One could add the engraved images found in 'pre-historical' caves, or the ancient monuments that are still a mystery to us. A few examples would be: Stonehenge; Easter Island; Nazca lines of Peru; and many more...

Ancient civilizations left us some truly remarkable monuments and artifacts. Some of them are unexplainable due to the degree of technological advances necessary to build them. Some, even to this day would be very difficult to build with our technology. The question is: Who built them?

We assume that the World has only made technological advances, and never regressed. But is this really so impossible that the only answer we have is:
Aliens built them.
If Aliens built them why are they not present on our World today? When advanced nations colonized primitive natives' lands, did they leave and never returned? The logic to assume such idiocy is simply nonsense.
Non documented History
Traces of not recorded history abound, just as much as disregarded recorded history.

Throughout the ages we have proofs of natural disasters that occurred along historical time lines: Earth Quakes; Volcanic Eruptions; Floods; Tectonic Plate shifts; Fires and dramatic Destructions; Ice ages...
Examples of disregarded history:
Mythology (we call it as such, but the people then probably did not call it that). It is however a very convenient name for us because we can't explain it.
Biblical Records and other scriptures, even though they seem to explain many things, they are an inconvenient reality, that the modern world has come to dismiss for ease of direction.
Mythology speaks of super humans or Gods, powerful creatures and giants.
The Bible speaks of humans living centuries, almost a thousand year in sone cases. It also writes about Giants and super strength (Samson). It also records a man building a huge boat 510 feet long in his back yard with his three sons (it took him a few hundred years to do so, and he must have been a master carpenter with great tools).

Could Mythology and Bible records speak of similar things?
If indeed such Men and creatures existed, what happened?
Well, the Bible speaks of a The Great Flood where everything on Earth perished, but those in The Boat and the creatures that lived in the water. The terrestrial World as we know it is only 4365 years old. Archeology and Oceanography are in the process of many discoveries. We still don't know much of our history, and of our Home, but progress are made, and there is still so much we don't know.
What could have been the population of the Earth in those times? How many died? Did the water recede to the same level as it was? What did remain under water?
That the Waters receded to the same level is highly unlikely since there are many underwater constructions that were found in this Century. The Sea creatures have probably remained the same.

To state a few mind boggling Ancient constructions and discoveries:
Egypt (pyramids, temples, artifacts as a battery)
Mexico and the Aztec constructions
Peru (Machu Pichu, Lake Titicaca underwater city, Nazca Lines, Aircraft like ancient artifacts)
Noah's ark at Mount Ararat (2 pieces)
Easter Island
Stonehenge
The mounds in USA
Underground city of Derinkuyu in Turkey
I suspect many underwater discoveries will be made.

Archeological discoveries
We still have only brushed the surface so to say, considering that the deepest dig ever done is merely of 7.5 miles underground. The mysteries of our Earth are still buried, and we can only hope that some day we will know a whole lot more. Sure, science has many theories, and Science fiction writers, give us so much to look forward to.
We still haven't figured out why the Arctic and the Antarctic are so different. One has life, the other doesn't. Maybe some day, Governments of the World will allow men to search this still undiscovered area.