(or some other Lesser Canyon)
I'm playing with story ideas for a videogame that takes place in the post-post-apocalyptic US. Everything is overrun with monsters and ocean horrors and whatnot, you've heard this story before. What happened to humanity isn't important, I just need a place to stick the remainder of it to start the game from. Initially this was going to just be a typical Last Remaining City but since it's essentially only exists to supply you while the entirety of the world is completely destroyed, I'm considering just shoving the entire thing out of sight and out of mind rather than trying to explain why this one city was somehow not leveled like everything else. The basic version of this is well-trod territory here, but I haven't seen anyone considering alternative methods to create shelter underground.
The Grand Canyon is (at its max) 277miles long, 18mi wide, ~1mi deep
Manhattan is 13mi long, 2mi wide, and 1WTC is 1,776 feet tall
So we can obviously fit several cities in there, but could we enclose them? The Grand Canyon is a bit excessive, but perhaps some smaller canyons/valleys that have flatter bottoms with no rivers running through them. Obviously this world is filled with all kinds of unobtanium tech, but I'm trying to start as much as possible grounded in reality and only apply phlebotinum as needed to scale things up. So would it not be easier to enclose a valley than to dig an entirely new underground area?
- The largest suspension bridge in the world is the Akashi Kaikyō Bridge at 2.4 miles with an uninterrupted central span of 1.2 miles. Impressive! Of course a suspension bridge would not make a great roof.
- The longest arch bridge and cantilever bridge are both about 1800 feet. Also probably not a great roof!
- If reading underground city questions here taught me anything, it's that you need a dome. The largest dome in the world is 310m. Not huge, but nothing to sneeze at! Perhaps a bunch of intersecting domes supported by columns?
- On the other hand, there are always massive salt mines, though those are limited by the salt deposit size. There's also a massive underground city large enough to house 20,000 people built in the Bronze Age, so maybe building underground isn't that unreasonable after all!
- Another alternative would be to modify a large existing cave complex like Jewel Cave in South Dakota
NOTE: In terms of the story, nothing is specifically trying to destroy this city, either during construction or after. It's more of a "we're not the top of the food chain anymore" situation.
I guess it's just really funny to put a roof on the Grand Canyon instead of yet another underground city so I want to know more about how unrealistic it is!