While the Durai Papers do not say a great deal more, they do mention that the land upon which the legendary city was built was severely limited─an inconvenience that forced its inhabitants to build up, rather than out.
Which led me to ask myself: what if the reason land was limited was because the land in question was an island?
But that still would not explain the omission of the cataract. It is inconceivable that the authors would neglect to mention so prominent a landmark had it existed. And then I found it─a lone passage regarding Goug's fall. While the particulars were missing, it appears the city was abandoned after an explosion left half of it in ruin.
I believe that this explosion may have undermined a portion of the seabed. It need only have been a small area at first─one that could well have gone unnoticed during the exodus. As time passed, however, the waters would have continued to wear away at the rock, slowly widening the gap until...
Well, until we have what lies beneath us today. This would explain why there is no mention of Ridorana prior to the present era.
...Come, Father─will you not tell them the rest? Of the possibility that we Garleans are descended in part from the displaced citizens of Goug?
It would explain how the Empire was able to forge its armies of iron even before the discovery and application of Allagan technology.
And it may also explain why the legend of Ivalice has resonated with our people for centuries, despite the fact that the lost kingdom was, as far as we can tell, in quite a different corner of the world.