Please don’t do this.
I work in a museum. A year or so ago, a time capsule was discovered in town and it was brought to us for opening and cleaning/preservation of the stuff inside. We were hugely dissapointed when it contained newspapers and various ephemera from the time period. We already have newspapers in our archives-- about two hundred years’ worth, and we get another copy every day.
What we were hoping for were* personal* items: letters, diaries, personal accounts of the events of the day. Magazines and stuff like that is a dime-a-dozen to historians. What we want (and what people of the future will likely want) is personal writings, family photos-- things that have* meaning.* Put stuff in the capsule that most likely won’t make the “permenant historical record” like your kids’ report cards, papers they’ve written for school, a note that came in a Christmas card from your Aunt Judy, a letter explaining what your hopes for the furture are, or how psyched you were to see your favorite team win a championship. Whatever-- just something that says something about YOU, your family, your times, and how you lived in them.
I know it’s a bit Captian Obvious, but make your capsule out of materials which will not break down over time. A stainless steel box is a good place to start. Inscribe on the lid “TIME CAPSULE” and date it (it’d be even better if you would do so on all sides of it) and engrave the lettering or it might wear off. The traditional opening time is 100 years, but if you want to open it in your lifetime, I’d aim for twenty.
Wrap the items in archival-quality materials, or the items inside could be ruined over time. Put any paper in acid-free paper envelopes, and put any other items in archival plastic (ordinary plastic off-gasses which can stain or damage items with which it comes in contact.) Seal the box as tightly as you can. Water is the worst enemy to time capsules.
Some items have what we museum folks charmingly refer to as “inherent vice”, meaning that whatever you do, given enough time, the item will decay because the materials its made of are unstable. Most paper of today is full of acids which will cause it to crumble into dust within 100 years. (If you’re going to write a letter or diary for the future, use acid-free paper.) Photographs will usually decay unless printed with special inks on special paper.
Do not put any mothballs in the box with the items.
Putting it in the crawl space is an excellent idea, as long as the area is dry. If you want to bury it, I would suggest packing the box contents well against moisture, and then wrap the outside of the box tightly in plastic (or seal it in a bigger box.) Bury it at the foot of a large tree, and put a sign above it. “Buried on this spot is a time capsule to be opened in 20–” You could make one yourself, or you could have a brass sign made. (You can go to Lowes or Home Depot, and back in the doorknob department, there’s a place where you can order metal plaques-- the kind used for addresses and “This House Was Built in 1850”. You can put on it whatever you want, priced per letter.)
