And you don’t want to annoy the things that live in the sewer! It’s not all waste and runoff down there, though most of it is pretty nasty. From the pet crocodile that got flushed near the wizard’s academy to the degenerate humanoids even morlocks avoid, the creatures dwelling beneath the city are often worse than the ones trying to break down its walls. The “benefits” of civilization include sentient rat piles that worship disease, grumpy hybrid chimeraling familiar rejects, and the flashing light of the prismatic cube!
Of course, that means there’s treasure down there too. And how bad could it really be?