The source of this activity may have been my fascination with fossils. I always told anyone who'd listen that I wanted to be a Paleontologist, which was easily the biggest word I knew when I was 7! It may have also been the frequency with which the Blue Peter TV presenters seem to bury Time Capsules in the BBC garden [are they still there I wonder?].
My own particular thing was burying coins. Old pennies, threepenny bits, the odd US cent and some Italian Lire all bit the dust. But by far the biggest coinage I deposited in the earth were Spanish Pesetas, which came back with my Parents from their Spanish holidays [why didn't they take me I often wonder? Maybe they thought I'd bury their foreign exchange!]
My cache of coins were always buried in an empty biscuit tin on the sloped end of our garden. A sort of a small hillock really - the slope, not me! Maybe a couple of years later I would dig them up to check on any visible signs of decay. The tin had usually rusted through and the coins were usually fine except for the pesetas, which degraded into a sort of silvery flour if you snapped them in half. I'm not sure what they were made of - maybe biscuit?
I also buried diaries under my bedroom floorboards detailing various snogs and cuddles I'd had at the back of the Catholic Church with girls in my year round 1969/70. I never retrieved said diaries so I can only assume that they are still there having been thoroughly digested by various generations of mice!
Did you bury stuff as a kid or maybe dig things up?