Under. Below. Beneath. Lower. While all have to do with downwards, there are many meanings behind them.
The Undercity is hidden from most of society. Good folks don’t need to see how bad others have it. It is much better to hide it beneath the surface. This way, no one feels bad about the lower-class citizens living there. Who cares if they have no sunlight, no fresh air? No one, since no one sees it.
They deserve it. If someone’s in the Undercity, it’s because they’ve been deemed a non-functional part of society. That’s why they’re lower-class citizens, because they don’t contribute. They’re below what’s expected of a member of society. It is best for the rest to have them locked in the Undercity than to have them roaming the streets at night, making the good people feel unsafe. Figuratively locked, that is. If they try to leave, their bomb will go off.
Some would argue that those living in the Undercity have a debt to make up. They’ve done wrong, and they need to make it right. Whether they do it by their own free will, or through an external agent, it’s all the same. There’s a life-saving movement that acts in the Undercity. They take what’s needed to serve the good people of the surface: kidneys, livers, cybernetics, hearts, you name it: this helps criminals to pay for their wrong-doing, and gives their life a purpose.
This might sound absurd if it were happening on the surface. In the Undercity, however, there’s no one looking out for the exiles. They’re trash, and so harvesters collect their goods, and then sell them on the surface to serve the greater good.