You don't even need to do that. Just bury it in the dirt, and nature will destroy or neutralise anything man has ever built. Even radioactives can be disposed of - the core of the earth is kept hot by them already, so it's not as if they're something we don't already have.
(Edit: except organochlorides. Earth will destroy these, but only once they've been floating around in our adipose tissues for a few hundred years or eating ozone.)
