The theory that I’ve always liked the best was that, what is now the Mediterranean Sea was once a vast fertile plain, which also happened to be below sea level. The huge rock formation which is now the Straight of Gibraltar gave way and the gushing Atlantic Ocean buried (“sank”) the civilizations that were sprouting up in this fertile valley. Atlantis was the most pronounced one.
Apparently there is geological evidence of some seizmic event that would cause the Straight of Gibraltar to crumble, and also evidence of a mass wash of fluid running through the Mediterranean valley. But the most compelling, was that on some Greek islands, especially Malta, you can see age old cart tracks and roads that lead right into the Mediterranean Sea, as if they were suddenly cut off by the water.
Actually, if I remember my college astronomy classes right, ancient Greeks who believed that the Earth orbited the sun were in the vast minority compared to those who believed the sun orbited the Earth. However, it is pretty amazing that anyone believed in a heliocentric model so early, as some (like Aristarchus) did.
There is rampant speculation that stories of this flood may be preserved as the story of the Flood in Genesis, in Gilgamesh, and the Atlantis legend. This is a speculative, and probably untestable, but also compelling hypothesis.
“The Encyclopedia of the Strange” by David Cohen mentions two aspects of the “Atlantis-as-Minoan Crete” theory that seem to have gone unmentioned as yet in this thread. The first is that most of the numbers in Plato’s stories, if divided by 10, match up pretty well with what’s known as Minoan Crete. It’s entirely possible that, assuming Plato actually got this story from some original Egyptian source (via Solon), he misinterpreted the Egyptian symbol for one hundred as one thousand. The second is that the purported location of Atlantis is not, strictly speaking, beyond the “Strait of Gibraltar,” which would place it somewhere withing the Atlantic Ocean. The term used in the original stories is that it lies beyond the “pillars of Hercules”, a term that described (in addition to the Strait of Gibraltar) several locations around the Mediterranean basin, including a rock formation in the Dardanelles which did face Crete.
I vote for Minoan Crete. I find the logic to be quite compelling.