As the oceans rose and weather patterns became unpredictable, populations retreated to high deserts and midland plains which were existentially boring, so it made sense to build cities with only a few windows. Rooms and halls that didn’t need windows or outside air could be deeply buried in the structure, level after level, kilometer after kilometer. Eventually, like the many levels of Troy, as megacities wore out, they were rebuilt on their own ruins. It was cheaper to build new on top of old instead of recycling decaying substructures. Meanwhile, criminals and cults tunneled into buried levels to support their nefarious ends.