Seeds has been mentioned a couple of times. How can seeds be kept long term? Could they be stored embedded in something like wax?
Bruce and others with more experience can probably tell you better, but it really depends.
As a general rule my understanding is storage is optimised by a cool (or even frozen) dark environment with low humidity and not too much oxygen (but more than none). That will act to slow down the low level metabolic processes that keep a seed "alive" enough to germinate.
However, you really need to go seed by seed - different seeds for different plants have different rules. Some are much better suited to longevity than others (moisture context plays a role, for instance). Some things (tubers) cannot be kept easily long term. All seeds will statistically lose viability from year to year - so you need quantity to mitigate against that (again smaller seeds are easier to store in bulk.
If you were literally burying them, waterproof sealed packets would probably last a fair while, but be wary of degradation of the packing materials - plastics, metals, you need to consider longevity of that too (and resistance to pests).
Personally I try to keep mine in the fridge whenever possible, but might need to temporarily cache them sometimes.
You should also note that most seed banks of note will periodically do "grow outs" to rejuvenate the seed stock. If you're hoping to cache them for 50 years, that's pretty ambitious for most
seeds in my opinion... perhaps even absurd for many (in the storage conditions postulated).
Just to make life even more complicated, not only do you need to consider the ability of the seeds to retain viability over the timescales in question - you also need to consider the utility of your choices. Hemp or flax might be more valuable than radishes and turnips, for example. Don't just think food - seeds can produce all sorts of other things - materials, medicine, etc.
Who will know the location and how will they recover the cached items/seeds? Will they know what to do with them and what conditions are suitable for each species?