I read your latest blog post about a creating a cache, and have a suggestion...
I am the proud owner of 2 genuine Army ammo boxes. Metal case, rubber seal, and the lids lock down like a vise. These things are incredibly sturdy, and though they are a bit hard to find, are well worth the search. There are many different sizes, but the easiest ones to find seem to be the 5.56mm 10rd magazine boxes. If you loaded one of these up with gear, gave it a coat of waterproof paint, and buried somewhere, I imagine it would be decades before the seal would fail.
Thanks James, totally forgot about good old ammo cans. They rust pretty quickly though, specially the ones that have seen a bit of use.
The rubber seal must be in excellent shape, and a fresh layer of anti-rust paint wont hurt. I'd also add a bit of grease to the outer surface and seal it airtight in a thick plastic bag.
For an underground cache I'd prefer either a sealed bucket or PVC tube.
For those that are extra paranoid, ammo cans should provide EMP (electro magnetic pulse) protection, so you might use one or two for electric equipment such as radios and other gadgets. The metal gun safe should also work as well.