Not sure if I'd call this a conspiracy. Some certainly would, others not so.
There's an increasing amount of compelling evidence that there may have been an advanced human civilisation thousands of years before the accepted time line.
There are monolithic sites which have been found that predate the believed building of the pyramids by many thousands of years at a time when man was supposed to be no more than primitive hunter/gatherers.
Further to this, there is a lot of compelling evidence of a large scale global cataclysm around the time that may have 'pressed the reset button' on man kinds progress. The most popular theory is an arctic meteor strike causing global flooding (a possible source for the great flood myths found in many religions and cultures today)
Of course, the loons (Ancient Aliens, Atlantis believers etc,) are all over this, but there is some genuinely compelling evidence coming through.
There is a lot of resistance from a lot of historians though and subsequent mudslinging. Some historians/researchers saying this is 'psuedo-science' and the other side accusing the academics being ignorant in order to protect their careers.
Almost every known ancient culture has two shared elements, a flood and gods/giants/great men of stature who came from somewhere else, not necessarily off planet, but other lands, depicted as 'otherworlds'.
The sphink, conventionally dated circa 4,000-5,000 years old, has been found to have water corrosion dated 10,000 years old.
Gobekli Tepe in Anatolia is dated at 12,000 years old. Baalbeck and its huge monoliths 8,000 years old, all man made. The peoples who made it vanished.
To read about these cultures look for Uriels Machine, detailing the movement of the 'gprooved ware' people from the Hindus through Europe, Scandinavia into Britain and Ireland. They brought knowledge with them not see before, structural, mathematical, cultivation and horticulture.
Secondly look up From the Ashes of Angels, compelling stuff on ancient culture in Anatolia again, how 'priests' moved into the mountains after the flood and came down to primitive survivors and taught them writing, metal working, medicine, star mapping and many other things. These stories are repeated in ancient scripts from Sumeria, Babylon via gnostics, Chaldeans. The Enuma Elish, the Epic of Gilgamesh, the Mahabarat, all tales of superior beings from other lands, exiled by cataclysm, teaching technology and introducing foundations for not just the societal structures we have now but the dogmas, hierarchies and structures imposed by some of those wise ones abusing the knowledge and forming religions to control. In that element is the story of the fallen angels or the Annunaki of Sumerian legend.
The cataclysm itself is recorded as a flood, the cause of the flood is probably a combination of a meteor, though as the flood is detailed from cultures right across the globe, this is open to debate, unless it is further back in time than is considered now, and localised to a smaller more advanced people who then took time to migrate over vast distances.
Another option is a pole shift, a natural phenomenon that has occurred periodically, and shifts both poles with ice caps melting rapidly before reforming at the new points.
The information supporting this theory is out there, but not 'accepted', it is never discussed or talked of in the mainstream, and the cultures to which the tales belong have long since had the interpretation skewed to mislead.
But it is something I have believed for many, many years, and I've spent the majority of my life researching it. In my opinion it is far from conspiracy.