Stone, or perhaps clay. Some of the oldest written records we still have are from Sumer using cuneiform on clay tablets. Use modern micro-abrasion techniques to

Stone, or perhaps clay. Some of the oldest written records we still have are from Sumer using cuneiform on clay tablets. Use modern micro-abrasion techniques to encode data to stone, then put that stone away into an area little accessed by humans on a daily or yearly basis. Assuming there is no cost limit here, I would go one step further and say use some form of metal. Say stainless steel, aluminum, gold, or titanium. Some metal that is very stable over time and does not interact with the atmosphere readily. Again, use micro-abrasion / carving technology to write data to the materials. The next question is what format for the data? It depends on whats being stored. The biggest issue is that of "formats". Lets look at things that last a long time. The English(or any language) is unlikely to change that much in 50, 100, or even 200 years. Words and their meanings will change, but for the most part a native English speaker 200 years from now could read what we write now. Whether or not they understand the usage of the language is a different question. So if its a textual document you're saving, write it in plain English. No abbreviations, etc. What about media? That gets complicated. If its a static image, perhaps keeping it simple is best. In plain English, write that the following section of data is an image. Each group of three numbers starts at 0 through 255. In procession from left-to-right, the values represent Red, Green, and Blue. Each group of 3 numbers is what we called a "pixel". The image is 300 pixels wide, and 800 pixels heigh(arbitrary numbers for this argument). For moving images, further expand on the single image description and say every 24 images should be spaced equally and viewed over the course of 1 second to achieve animation. Sound is something I don't know anything about from a data format perspective, but I would again find the simplest mechanical way to produce a sound and store it in that format with ample verbage describing how to handle it. Edit After reading other responses that came in while I was writing this, I want to add some more thoughts. Remember that our technology is ephemeral. We don't really use much tech from 50 years ago, hardly any from 100 years ago, and it just gets worse from there. Things like microfiche, ssd's, cd-roms, blue-ray, etc are all the more bad ideas for long-term backup. Paper books are a better option than any of these for near-term storage for time periods up to 50 years. If we want to actually store data in a meaningful way for long periods of time, say over 100 years, we have to keep it simple. Your devices will probably not last 100 years, even if kept in storage under the most secure environment. But in 100 years people will still have eyes, ears, and hands. We have to look back over history and look at the material types that survive long periods. Stone, and metal to an extent, are very good long-term materials. Cloth and paper are not*. DNA is potentially a usable data store, but is corruptible. Plus you can't read DNA patterns with the eye.