The Y0 interlude refers to the apparent gap in history around the year Y0*, as measured by the progression of the era clocks. Exceptionally little evidence prior to this time remains, whether material or otherwise. There are no physical sites that can be conclusively proven to have predated Y0, though limited evidence of a prior era does exist in intangible form.
What did survive the Y0 interlude is perhaps more interesting than what did not. Certain oral histories of the Flower Kingdoms, as recorded by their ishuuns and water-dancers, are believed to have been passed down from generations that lived before the gap. This is largely due to the mentions of astronomical events like comets, eclipses, and red suns recorded in the stories - as post-interlude histories of the Flower Kingdoms are equally precise as to the dates of such definitely known events, some scholars have concluded that they must have been equally accurate before. If so, the peoples of the Flower Kingdoms, or their predecessors, not only predate the interlude but do so by a remarkable degree, at least another twenty thousand years.
Followers of the Draconic pantheon often believe the Hidden Moon, Udsakar, to have disappeared around the time of the Y0 interlude, which might be supported by the histories of the Flower Kingdoms.
Certain still-alive individuals, such as Anlabban, are also thought by some to have survived the interlude.
There is no generally accepted theory as to what caused the interlude, or what existed before. There isn't even agreement that one occurred to begin with. Amongst those who do believe it to have been real, common hypotheses include as to what happened to those who lived before include an etheric ascension of the sort told in the stories of the Dwer pantheon, a massive cataclysm, or a mass departure of Kishar.