Today, one of Earth's many overbearing problems is overpopulation. The solutions are many. One of them is building new cities underwater.

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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.

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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?

  • $\begingroup$ Most new construction does interfere with old infrastructure. It tunnels through, routes around, or outright replaces it in a manner conducive to the new construction. Why should the existing tunnels not be touched, especially knowing that within an order of magnitude suggests it wouldn't be more expensive? $\endgroup$ Commented Aug 10, 2016 at 4:40
  • $\begingroup$ @NathanielFord I don't follow you. $\endgroup$ Commented Aug 10, 2016 at 4:51
  • $\begingroup$ As you say, the network under cities is vast and complicated. It's unlikely you could dig anywhere without hitting something. To adequately answer this question, the reason you're avoiding touching the current infrastructure is important to know. As the one answer currently points out, the only empty area is below - but more creative answers are possible if we know what we're optimizing for. $\endgroup$ Commented Aug 10, 2016 at 4:56
  • $\begingroup$ We're optimizing for underground cities of this magnitude, youtube.com/watch?v=yVkyfC_AOLI, except that this documentary doesn't cover the implications of already-existing subterranean infrastructure. $\endgroup$ Commented Aug 10, 2016 at 5:26

2 Answers 2


You go deeper.

You just have to go deep enough that you avoid the already built system.

There are also many areas which have nothing under them at the moment that could serve as entry points to your underground cities.

I imagine that an example would be having an entry point at the base of a mountain or in a forest somewhere, and it goes straight down into the first "city hub", which provides paths to a bunch of other underground cities. By limiting access to the cities and forcing people to enter/leave through the hub, you keep the surface clearer of holes as well.


If you want access to the underground city without first leaving the aboveground one, your best bet would be using the land of a park or similar, if available.

If it is not? Find the neighbourhood that's cheapest to buy up and least likely to have militant inhabitants, and dismantle it entirely. It shouldn't be too hard to find one that doesn't have any major "sewerage" lines routed THROUGH it, and any that were needed by that neighbourhood can be closed off without too many issues.


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