Three hundred and twelve years ago, many bombs rained from the sky.  When they hit they exploded, throwing up dust and setting the buildings and trees on fire.  The smoke and dust blocked the sun, and the ground froze.  Eventually, however, it started to subside, and people left their sanctuaries to conquer the surface once more.  Though some of them had old world tech and weapons, few could repair, build, or even maintain it, and within a few decades and working gun was rare, and bullets for it rarer still.
-Metal Doctor's history lesson to the children of 3'st'bway Sanctuary.

The people of Threest'bway Sanctuary live under ground, with old world technology which maintains itself using their salvage and what they mine.  Metal Doctor is one such example of old-world tech.  He is a medical robot that, after decades of interacting with humans, learned to act as one, and now lives among them as their healer and a teacher of medicine, old-world history, biology, and the like.
That is where I was born, though you may have been born elsewhere.  I was raised underground by the elderly and the women, and when I was sixteen I was sent to live on the surface, as all men are.  The women of the Sanctuary, though, spend much of their life under ground, caring for their children, and only coming to the surface to baby-make with the men.  Some of the women cannot baby-make, though, despite trying, and live with the men on a more permanent basis.  The men salvage, trade, hunt, and forage, and give it much of it to the sanctuary, and the sanctuary gives them what they can spare.

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My character will be banished from Threest'bway (3rd St. Subway in the old world) for one year while they investigate a murder he is accused of, and during that year he will learn of the surface, and discover the cause of the Day of the Bombs.