What if a person dug a hole in the side of a hill, lined the bottom of the hole with lead sheet, lowered a shipping container into the hole, draped lead sheet over the container, then buried it with the door exposed at the edge of the hill. Then line the interior of the door with lead. Seems like a good place to store dolor panels, dry solar batteries, inverters, transformers, controllers, a couple of new small simple refrigerators, (that are proven to run on said solar equipment).
and some gun safes!
Theoretically speaking of course.