Update: Check out paper-internet.appspot.com
If you wanted to preserve important bits of our civilization for future centuries, you could do worse than a bundle of paper sealed in plastic. It's remarkably cheap and effective; you can make one over a weekend. In this article we'll build a 1/2 scale model of a time capsule that contains the complete Linux 0.1 source code, plus sundry articles and internet ephemera.
A time capsule must perform three basic functions:
So while the Rosetta Stone performed (2) fairly well, it was pretty lucky to be found at all. Also its data density is terrible: about 1 bit per cubic centimeter. A book in a library fulfills (1), but requires the library around it to provide (2) and (3).
The internet, contrary to popular belief, is not very good at preserving information on a long time scale. It ultimately depends on digital media that break down rapidly. Early Unix source code, one of the most important sequences of bits ever written, had to be reconstructed from printouts.
The makeup of our capsule is simple: cellulose, carbon, polymers, and distributed information. You print a bundle of paper, place it inside a box, stick a label on it, then drown it in translucent epoxy resin. Alongside whatever it is you are preserving, you include the locations of other capsules.
The humble piece of paper has come a long way in the last few decades. Acid-free paper  is the norm. It has archival properties comparable to cotton rag or parchment, and can easily survive for 300 to 500 years. Black & white laser toner is carbon powder and resin, fused to the surface at a few hundred degrees Fahrenheit. Carbon, being an atomic element, never fades. All in all it's a cheap & cheerful way to preserve data for a very long time.
You need an airtight seal that is itself fairly rugged. Epoxy resin is the hard plastic you often see protecting the surface of bars and restaurant tables. It's the closest thing you can get to man-made amber. Our scale-model capsule will be encased in a shell of resin about 1 centimeter thick. We're not shooting for 2,000 years exposed in the desert, just 50 to 500 years in the ground.
The biggest design problem of traditional time capsules is that people forget where the damned things are buried. There seem to be two contradictory thoughts going on at once: that the best way to preserve information is inside a buried box, but that the best way to preserve information about the box is somewhere else.
Inside each time capsule will be a list of other known capsules. That, I hope, will make the difference between a node in a network and a forgotten box of junk. Dozens or hundreds of people could build full-scale capsules like this and share location data with each other. This prototype and its twin are the only two of their kind so far, so they only link to each other. The larger the network, the greater the chances of recovery.
Gather whatever data you want to preserve. It can be books, songs, computer programs, your Facebook page, diary, recipes, anything. I would focus on things that are likely to disappear. The future will probably most appreciate a description of boring, everyday life in Right Now, A.D.
Laser-print your data "4up" and single-sided. You should experiment with your printer's capabilities, but I've found that 10pt Helvetica printed 4up is the smallest mine can go and still be legible. Don't print double-sided because the toner might stick to itself if it ever gets too hot.
Cut the sheets into quarter-pages, collate them, then tightly wrap the bundle in a couple of layers of paper, like a Christmas present.
The box is mainly for appearance's sake, and to protect the paper from light. You could probably sink your bundle (wrapped in a few layers of paper and plastic) directly into the resin and it would work fine.
I made mine from illustration board. Cut two 12 x 15cm pieces for the top and bottom. Then cut the side walls 3cm high and slightly shorter than the 12 or 15 cm, to account for the thickness of the neighboring wall.
Glue it up! It doesn't have to be pretty or perfect as long as it fits as tightly as possible around the paper bundle. Let it sit for an hour to dry.
Place the bundle inside a ziplock bag, squeeze all the air out, and seal it. Put that inside the box and glue the lid shut. Paint it if you want, then glue a label to the front so people know what it is.
You build the mould the same way as the box, just 2cm larger in each dimension. I built mine out of balsa wood. If you have an aluminium or plastic tray of the right dimensions you can use that instead.
Needless to say, do all epoxy work in a well-ventilated area and follow the safety instructions. Epoxy resin sounds tricky but it's pretty easy to handle with practice. There are many types of resin of varying properties. You want "encapsulating epoxy resin" or "clear casting resin", which is often used to seal electronic components and art projects. The strongest resins take 48 hours to harden completely, but last much longer than fast-cure resins.
Mix & pour about 3/4 cup (140ml) of resin in the bottom of the mould and let it cure for about an hour. This forms the back of the shell.
Center the box inside the mould. Mix & pour the rest of the resin on top of the box, and let it flow into the sides. Loosely cover and let cure for 24 to 48 hours. Your inner box will probably not be water-tight, so expect some bubbles to stream out of it as the epoxy seeps in. (To avoid this you could use an airtight tin, though there is a chance it will float!)
Place in ground, let stand 300 years.
This is a scale model to demonstrate the process. Real capsules will contain at least 500 full-sized sheets of paper. The magic of the square-cube law makes it more cost-effective as you scale up. Casting large volumes of epoxy is a bit tricky, so start small and ask your friendly neighborhood supplier for advice.
Three Reams (1,500 sheets): This is probably the most managable size for a weekend project. You could use ready-made "archival" boxes for the inner box and one of those plastic file-folder boxes for the outer mould. Artist-grade resin starts to get pretty expensive at these volumes, but you can use amber encapsulating resin instead, about $80 for two gallons. AeroMarine sells in bulk and will send you free samples.
Carton (5,000 sheets): If you want something with more volume and durability, you can use a concrete flower pot whose inner dimensions are about 2 cm larger than the outer dimensions of the paper carton. Tightly seal the paper with several layers of thick plastic, and pour the resin as before.
Oil Drum (28,000 sheets): if you have good concrete molding skills and access to bituminous resins (instead of expoy resin) for the water seal, you could build a time capsule around a 55-gallon oil drum with very good capacity. It's probably wise to invest in higher-quality printing at this scale, so you can fit more than 4 pages of data per sheet.
Monument (3,500,000 sheets): A typical twenty-foot cargo container can hold over 700 cartons of paper. Constructing this monument requires a proper concrete foundation, a steel-reinforced concrete shell, and serious seals against moisture. The curation and printing alone are jobs of unusual size, but doable.
A cargo container sells for about USD$3,000. A contractor friend of mine estimated the concrete construction work would cost about USD$15,000. The printing, curation of the data, and mosaic work would be the most expensive items. But I believe the total cost would be under USD$100,000. That's less than many corporate sculptures, and a lot more useful.
I think it would be beautiful to put giant concrete archives in public parks around the world. A mosaic on the top surface would describe what it is and what it's for. Sink them about 2 meters down so they stick out a bit. They would form large benches for people to sit and play on, trace out the mosaic with their fingers, and perhaps be reminded of time.
If you want to build a time capsule yourself, send me an email! Let's get this thing started.
 "Acid-free" is a bit misleading. All wood-pulp paper contains acid that will yellow and destroy the fibers over time. "Acid-free" paper is given an extra wash, then impregnated with alkalies (baking soda, more or less) to improve whiteness and neutralize remaining acids. The percentage varies from 2% by weight up to 4 or 5% for "archival" quality paper.