I realize there's near zero probability, but the mention of mysterious Japanese gold made my mind immediately go to this:

https://en.wikipedia.org/wiki/Yamashita%27s_gold

That is a bad idea. Hydrocarbon polymers like PEX, ferrous alloys, and concrete would be much more practical.

And that's Osaka. Osaka's population peaked around 2017.[1] The only major city in Japan not on a downtrend is Yokohama, which is in the Greater Tokyo area.

Keeping up all the infrastructure as the population declines is tough. That's one of the challenges of this century for the developed world.

[1] https://worldpopulationreview.com/cities/japan/osaka