Always read the comments. On a slashdot story about "what do you want to happen to your data when you die?" (they're geeks, and this is important to geeks), I found this comment all about how fireproof safes and data vaults work:

Document containers consist of two thin layers of steel, which have a hydrated compound stored between them; ... Upon heating, the hydrate gives up its water, flooding the inside of the container with water vapor.

Media containers should follow the same general rules (be careful where you put it, etc.), but work on a different principle. Last I checked (it could have changed), media containers use wood as insulation.

Lots of good advice too!