Post History

## An alternative
Maybe they did notice the war had ended, but still chose not to leave the safety of their underground society. Over time, this alternative reason for staying could drift into the situation you picture, where most of the society is unaware that returning to the surface is a possibility.

## Fashion and prejudice
In our world, throughout history, each generation has complained about the next. It only takes a decade or two for many people to start making criticisms of the changes that have occurred, even those changes that are little more than fashion.

How many of these have you heard?

> People today don't:
> - Recognise good music.
> - Know how to dress.
> - Respect their elders.

None of these things are unique to recent generations. People have been saying similar things for thousands of years. Fashion is part of human instinct, and groups of humans have always settled on arbitrary patterns of behaviour, treating those who differ with suspicion and disapproval.

I would expect this alienating effect to be even stronger when your entire society is just a few thousand people. (The size of the underground society is not specified in the question - this may affect the likelihood of the situation described here.) The differences are also likely to be significantly larger after the two populations have been diverging for centuries rather than just decades.

There are likely to be exceptions - people who are fascinated by how different the outside world is, and want to go out and experience it. However, this may be seen as a threat to the society's "respectable" (currently fashionable) way of life, so even those who want to go out and embrace the new ways of living are forbidden from leaving. Many may feel that the comfortable life underground with plenty for all depends on keeping it secret.

This may lead to prejudice that restricts what people can say or do beyond just preventing them from leaving. If leaving is illegal, then dressing like an overgrounder, or even using the colloquial language of the overgrounders, may be disapproved of at first, and later even punishable by fines or imprisonment. A small society with a history that started with an extinction threat (a fear even worse for many than personal death) may make undoing such harsh laws difficult.

## Insular policy
Resistance to laws preventing going outside may lead to gradually stronger laws, at first preventing emulating the overgrounders, and eventually forbidding discussing them at all. As a result, access to the recordings of the current state of the outside world may then be restricted to a few people in authority, and as generations pass the general public may have no knowledge of the outside, beyond whispered myths of "those dangerous people with their constant wars" even though the surface has been largely peaceful for centuries.

Each new set of people in authority may choose to continue this situation due to sharing the prejudices that have arisen, or out of a belief that the society is better off here, or a desire to maintain power - ruling the world is easier when the whole world is a small underground society.