We do need to look off the coasts. Apologies if this posting offends your rational sensibilities.
As you already know, "megalithic" is a combination of two Greek words meaning ”large” and ”stone”. In anthropology, the term refers to giant stone constructions erected in prehistoric times, often by unknown people. Also, Incan sources from centuries ago attributed their huge stone structures to an ancient, pre-Incan race of “giants”.
Esoteric lore conveys that the typical Atlantean was about ~2.6 meters tall (8.5+ feet). Since that was ‘average,’ many were taller. (BTW, since they carefully observed Nephila, or Orion, this is likely how “Nephilim” became synonymous with “giants”.)
Collaborative European research (between the Universities of St. Andrews, Aberdeen, Birmingham, Dundee and Wales Trinity St. David) has used underwater archaeological methods to show that humans thrived more than 12,000 years ago on a “lost” dry territory called “Doggerland” that connected areas around the British Isles to France and the Lowlands and even connected Scotland to Denmark before all those plains were submerged by rising ocean levels.
Those now-undersea areas have been colorfully described as “northern Europe's own lost city of Atlantis” [see the following link from my books [www.cbsnews.com]]
And that's my two cents worth!