Reasons for a society living underground

I'm basically writing a post-apocalyptic novel, set around the decendants of a group of survivors who live in an underground city. The story would be happening at a minimum several hundered years after the event. I'm working on the basis that they all sealed themselves inside a military bunker containing hydroponics equipment, and so managed to survive the immediate aftermath so they could then expand.

The question is what event could drive them underground, leaving them in a technologically superior position to those on the surface after the event? I basically want the earth driven back to at best medieval conditions, with more advanced technology severely limited, with only a few societies on the globe in posession/have knowledge of it and very limited post-1900's equipment.

Effectively, what I want is a reason for them to need to sit in a bunker for several hundred years and long enough that they choose to remain living underground once they have the option of going above-ground, with enough time that surface society has been decimated, lost the ability to make industrialised (post 1700) equipment and has fragmented into small kingdoms/tribes.

Any thoughts?

Fett d'Facto

1) Well on the generally terrible TV show Jeremiah there was a disease that wiped out anyone over a certain age. Thus one group of scientists and soliders in a military bunker survived but had to stay isolated because they were not sure if the virus was actually wiped out or just dormant. They then started abducting people from the surface to conduct tests to see if they were clean of the disease. If I remember right no one had a true immunity. The disease only killed adults. In infected children the virus just died out. The disease simply burnt itself out. Everyone on the surface was scavenging to survive. Remember that EVERY ADULT on the planet died within a short time. Modern civilization did not have a chance. The surface group with the good stuff was run by a childhood prodigy but was still just refurbishing leftover tech. The bunker survivors had attack helicopters with napalm and "bunker buster" missiles. The show ended (or I stopped watching..one of those) before this conflict reached its most likely one-sided conclusion. All this happened within a single generation.

2) If a large meteorite hit it could cause a climate change that would wipe out crop yields on a global scale. The resulting chaos would hinder trade and so on, leading to regional conflict. Depending on size the whole planet could suffer a short (or long) ice age. If it lasts too long it does not matter what any goverment does, a massive percenatage of the world will die. Your survivers were just waiting out the cold.

3) Some idiot releases a genetically-engineered something that only eats petroleum based products. Unfortunately such products, aside from actual fuel for tractors and such, are key ingredients in the fertilizers and other enhancers used to increase crop yields the world over. Farms do not become barren, but are only producing a fraction of their potential and the economy and poor suffer. Crime and cultural tension rises. Then factor in the supply chain found in much of the world, which is dependent on cheap or at least accessible fuel supplies to get A to B. Bonus points if the superweapon-thing mutates and spawns a race of monsters that continously evolve. Thus the survivers emerge and are beset upon by Red Slimes!

4) The aliens from Gor invade but then get bored and leave after reducing mankind back to pre-industrial tech. The survivors saw them pack up and leave, leaving human serfs behind. Thus it was clearly time to get humanity back on track, whether they like it or not...

They decided they just didn't like the surface very much. And there was more room underground.

Nuclear Winter or some such thing renders the surface uninhabitable for however long

Yeah, any sort of major nuclear...happening, would make it too toxic to live except in a sealed bomb shelter underground. With an air filter obviously.
They length they had to stay underground would depend on the magnitude of the bomb/winter. Hope this helps! :)

John Shirley had an underground city called the Arcology, which was started because the man in charge wanted to create a society that would be unaffected by the corruption of the modern Western world.

If the atmosphere was destroyed, and the population had to move into sealed underground chambers.

In the quite amusing film 'Blast from the Past', a family takes to their bomb shelter during the Cuban Missle Crisis, only to have a plane crash on their house and knock out their external sensors. So they stay down there for 30 years, only to emerge in the 90s. (The culture shock actually convinces them that society had indeed collapsed).

I mention this because it would be possible for a society to lock themselves away from some looming danger, whether the danger came to pass or not. (Or perhaps as in 'The City of Ember,' the people don't know to leave the protected area, even when the danger is passed). In both these films, only some in the group have any knowledge about how to determine whether it would be safe to go out, and if they are unwilling, a zealot, or convinced the danger still exists, they could keep people trapped in 'safety' far longer than what was necessary, in fact long enough that the resulting generations may not have any idea WHY they were down there or that this wasn't how 'it always was.'

Heating or cooling. Living underground, the temperature would be stable. If it is very hot outside, you get warmth from the earth. Perhaps the outside is just too hot.

My first thought is that they were religous zelots, thinking that the Great Tribulation was apon them and they could ride it out in the vault. They'll stay long after the surface recovered from whatever happened (even the fallout from a barrage of salted nukes won't last more than a few decades) out of religous belief that in spite of what the sensors are saying, the surface is still a living hell.

If you want to skip the christian imagry you could go with a heaven's-gate type cult that belives a cosmic horror was visiting earth on a comet.

Either way, you can have them go back to the surface after either a numerologically significant period of time, after some essiental system failed requiring them to leave or die, or have an internal schism of people sick of living in a cave for their entire life.

Come to think of it, I'm from Singapore and there are some compelling reasons to live underground: the sun, and a trillion mosquitoes.

The only problem is the technology. A number of malls are already subterranean.

Tradition? Religion? Perceived superiority to the "dirt grubbers"? Safety (harder for weapons to hit)? Aesthetic reasons (don't despoil the surface? Recall something like this from a Robert Heinlein novel, but not the name of the novel, not terribly applicable in your case).

Control? (e.g. Whomever rules builds subways etc but not roads, and you can't cut "cross country").

I tend toward a combination of tradition ("this is the way we live"), comfort (lighting, temperature, etc all regulated).

I'm sure you can figure some reason out; remember it doesn't have to make *sense*; "culture" covers a lot of ground.