It’s on Tural, the continent to the west of Eorzea.

It is physically on the new continent, and can be seen on the new map in the northern area. In the previous EU fanfest showing off the new desert zone, they had a massive [SECRET] mask on part of the image, which we now know to presumably be this city given that they showed it as connected to the desert zone via train.

We don’t know much about it yet other than that it will be the endgame “town” (Radz-at-Han, Eulmore, Rhalgr’s Reach, etc) and it is brimming with lightning-aspected aether.

My only guess for now is that it is from a different world: one of the other reflections of Etheirys (the Ninth, maybe?) which may have managed to transpose part of itself to the Source physically rather than just its aether upon a rejoining. Looking back to Shadowbringers, the Exarch’s explanation of the reflections displayed the Twelfth as the shard which was flooded by lightning aether and rejoined with the Source, while the Ninth still survives, but that could have just been his example to illustrate the concept without actually knowing exactly which shards were consumed to trigger the calamities that we know about.

They had mentioned that the name of the area (Solution Nine) is significant. There’s only one time before this that I’ve heard it used before, which was as an ability used by Zidane in Final Fantasy 9 when under the effect of Trance (that game’s version of limit break). In that game, there was a similar story of worlds merging which led to the features of one world becoming present on the surface of another. Maybe it’s a reference? Might be a bit of a stretch.