Today, one of Earth's many overbearing problems is overpopulation. The solutions are many. One of them is building new cities underwater.
Sub-Biosphere 2, one of the proposed underwater cities of the future.
Another option, relevant to this question, is to expand already existing cities underground.
A planned map of a future Singapore.
There are advantages to building cities underground. The temperatures are constant year-round, which make the underground preferable to dealing with merciless heat (Sahara Desert, Australian Outback) or seasonal extremes (Chicago, New York City).
Still, there are many problems with underground cities. The one for this question is one word--sewerage. Beneath our feet are already thousands upon thousands of miles of tunnels for plumbing, transportation and drainage. Any idea on how to build an underground city WITHOUT dismantling the existing tunnels in the process?