Thanks Openurmind. It turned up in a search for something else (I don't even remember what) and I'd never previously heard of it, and Googling turned up no occurrence on FR, hence...
Thank you for the article, it’s cool stuff... I always wondered if some of these underground cities might have also been somehow need for environmental reasons aside from security reasons?. Be interesting to try and see what suspected environmental science claims about these particular timelines of some of these.