Defiance is 200 meters above Saint Louis (How the frakk big is the arch? 630 ft (192 m) Thank you!).
The old city was buried.
Similar to Waterworld, maybe they're just mining for the past?
Their city is built on top of thousands of buildings and hundreds of thousands of homes and millions of cars and busses.
Why mine for new metal when you can just recycle the metal that's already been processed?
Unless Saint Louis wasn't buried so much as struck by a targeted meteor that is rich in valuable resources.