When you eliminate documented proof the result is 'educated theory.' That is like a few years work that I did in genealogy. There are branchless gaps in a family tree and the documents are lost, burned or non-existent. In such a circumstance an educated theory based on compelling circumstantial evidence is acceptable...
Acceptable but not proven. Perhaps in time further evidence will emerge that better explains the results.
That holds true of ancient unknown advanced civilizations: there are facts that could point in that direction but they do not comprise indisputable proof. I am reasonably convinced that trans-Atlantic commerce has occurred since at least the time of the Phoenicians - but it will be very difficult to prove it, especially in the condition of modern academic thinking. New ideas are consistently rejected and promising careers are ruined.
Such currently holds true in the developing research project taking place on Oak Island, Nova Scotia, conducted by trained archaeologists. Increasing evidence of European engineering works as early as the 13th century are being uncovered and confirmed by scientific data. Some evidence that dates back to Roman times... but no proof of how it got there or when.
On one of my own web pages I relate the unhappy story of the young archaeologist that discovered the first Paleolithic cave paintings in 1879 in a cave at Altamira, Spain - Marcelino Sanz de Sautuola. It destroyed his life and he died in shame and ignominy. It was considered foolishly shallow to consider that primitive man could discover paint pigments that could last 40,000 years - just ask any house painter.
Shortly after de Sautuola's death more Paleolithic cave paintings were discovered in France. To his credit, de Sautuola's main critic repented and became an avid researcher in the ancient artifacts - but Marcelino Sanz de Sautuola never knew it.
There are so many stories like that.