When it comes to city planning, “upward” and “outward” are never the only options. The most impressive cities are those underground, saving space by extending into the earth. Though this may sound far-fetched, such structures exist around the world, and can be easily found in:
United States
Having been inhabited for thousands of years by a wide variety of peoples and civilizations, the US has no shortage of architectural wonders. Among them are a number of different underground cities, some built recently and others millennia ago. These include: