i got a jar from asda with non metallic lid and i put all my laminated offline wallets from doge to btc in it and buried it 12 ft deep reinforcede concrete box in my garden with a digger and cemont mixer i hired
12ft underground? You can't reach 12ft underground with a digger. You would have to dig in levels, which means you would have dug up the whole garden of yours and where did you put the mud while you were digging?

Nice idea nevertheless.
Yaeh 12ft underground seems overkill, you would need to call professionals with huge machines which would alarm the neighborhood, not a good idea, it seems safer to hide them under the bed at that point.