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If they are acting rationally, it's not possible. It is only logical for them to monitor the progress of the situation that was bad enough to force them to move underground. Even if society was lar...
Answer
#1: Initial revision
If they are acting rationally, it's not possible. It is only logical for them to monitor the progress of the situation that was bad enough to force them to move underground. Even if society was largely uninterested in topsiders, there would be researchers, adventurers, daredevils, outcasts and criminals that would seek them out anyway. They could not stay in the dark if we're talking at least millions of people and hundreds of years. There might even be talk of coming back up after the combatants are devastated and collecting resources. Regardless, advanced technology implies they will be interested in Space, so they would have to go through the surface anyway to get into orbit. For them to stay uninformed to that level, there would have to be some kind of irrational mass hysteria. * There could be a strict religious prohibitions against interacting with the surface. There will always be some heretics who do it anyway, but the religious sentiment would suppress them and prevent word from getting out. * The government could follow an aggressive propaganda campaign suppressing the information, perhaps orchestrated by the elites to protect their power structure. This would be a bit like 1984... But then you have to explain how the state survives unchanged for hundreds of years, something rare in history. The state that inspired 1984, imploded after 70 years - 40 from the publication of the book. Either of these implies also a significant decay in the underground people's society. Decay alone (technological or social) would also account for them being uninformed. It makes it sound as if they are effectively some sort of doomsday cult, steadily shrinking and declining from the date of exodus, and would be irrelevant by the time of the discovery. The alternative would be if there was an unrealistic "new frontier" underground - like the Underdark of Forgotten Realms where you can endlessly dig deeper to find resources (including food, light, air, water) and living space, as well as magical portals to new, vast realms. In a more realistic setting like Earth, they would essentially have to find something like the buried gates from Stargate.