From the quest "Annihilation":
Mikoto:
According to the Zodiac Brave Story, the Clockwork City of Goug was a metallic metropolis wherein airships, automata, and other technological marvels of the age were first conceived. Little is said about its location, however, save that it was far away.
Jenomis:
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.