You could look at fire safe boxes for document storage. Those are usually pretty solid. You would want to bag up the drive inside an anti static bag and probably put a couple of those little water absorbing silicone packets in there as well. If access isn’t an issue then maybe some sealant around the seams to keep it more water tight.

Magnetic tape would be better for long term storage as well I think. Those have longer storage stability. I don’t know how long an unplugged hard drive will reliably store information.

Animals could dig it up but probably wouldn’t as it wouldn’t smell like food. Depth wise I’d go for at least a couple feet deep, the traditional 6 is a surprisingly deep hole and temperature gets more consistent the deeper you go (at least with readily available tools, it eventually starts to get hot again).

Please note totally random opinion with very little experience with long term data storage. Thanks for the fun thought experiment, I hope things get better and you don’t need your backup data.