number 6- yes, the site known to divers for years.

Trying to remember my school memories of Plato's bo
ok, I think that it was a fictional device to illustrate a point, as do some others.

There are sunken cities at several places around the Mediterranean and elsewhere. One site I read about in Nat. Geographic had been built in Greek times and submerged; the pillars were marked by marine organisms, and then the area was again uplifted, exposing the pillars again.
Much of the site of historical Alexandra is now under water. The earth is constantly shifting.