Big corrugated culvert and Quonset huts have been buried as temporary military installations. Steel rusts and aluminum and stainless are expensive.

If you want a waterproof container I'd look into used plastic or fiberglass industrial process tanks (preferably ones that held something not too nasty, so likely from the food industry), dig a hole, do proper compaction and gravel base then plop the tanks in (fill them with water or something so they don't float away) and then pour concrete around them for strength (leaving holes for your vents and doors and whatnot). You're going to have to become well versed in chemistry and construction to prevent all your "rooms" from leaking at the doorways where you have cut holes to join them together.