I ran across this book a few years ago, and really enjoyed it:
It discusses the legend of Atlantas from Plato's writings through the 19th century to more recently, and through several lands where historical accounts near the same time in history have similar stories of large, intelligent castaways, who were considered giants by the locals, appearing, telling of the distruction of their civilization.
Given that the history of the planet covers billions of years, I find it reasonable to believe that such advanced civilizations existed, and were demolished whether though human or natural intervention.
It seems entirely possible to me that the earth's gravitational force could have pockets where it is less strong, or in the history of the earth, it was generally weaker, and could allow people with larger bodies to exist in good health.
We still see remnants of gigantism in people who are born and grow up to have larger body frames than average. Perhaps they have some trace of the blood from Atlanteans. There could have been pockets of these large, intelligent people all over the planet that could have been weakened by the changes in the earth's gravitation, or had some genetic weakness that became stronger over time.
I do find it fascinating, and to speculate that the structures we find made of large heavy objects, the pyramids, Stonehenge, may have been built buy such people, for whom such objects would not have been difficult to move.