Hard disks and long (I mean long) storage periods. This question is strictly on Hard disks or any storage medium.

What storage medium would be the best to use if I intend to, say, zip-lock, bubble-wrap, stash in a steel chest, and bury my (100-1000 GB) data in a time capsule?  What would give me the best chances of digging my stuff up 15-20 years from now and being able to access my data?  

If not feasible, how good are current USB portable HDDs at keeping data for said periods of time?