Would it be possible to do something from imaging satellites — something akin to ground-penetrating radar / laser range-finding / ultrasound — that might not be good enough for much, but which would be "just good enough" to find any other gigantic cities with walls built of dense stone, hidden under 10-50ft of dirt or sand?
The Brits had started a Lidar survey of most of their country a few years ago, I'd say 2015-2016, but I'm not sure if that information is entirely accurate and, if it is accurate, I'm not sure how far they have got with it (what if all the cuts made to spending money on stuff that is not seen as essential).
I'm jealous in a positive way that the Brits have managed to do that. There was some talk about trying to emulate them when it comes to some archaeological sites here in Romania, but apart from a couple of tries/mappings nothing systematic has been implemented.