I’ve seen experiments with buried Conex containers. You have to reinforce the roof, otherwise the weight of the dirt will crush it.
My dad claimed he was making a “root cellar” out of one of those huge, used fiberglass gas tanks we can buy in this area.
I watched with amusement as he dug the ditch into the bank, carefully placed it and then...poured concrete over it for good measure.
Not surprisingly, it collapsed.
So, there went his fallout shelter....er....’root cellar’.