Verborgen of niet toegankelijke bestanden.

Google will often hint to links that are not suppose to be reachable by the public.

This tutorial will get a user started in using this information to gain valuable data regarding internal networks.
Directions - Searching google for site:nasa yields links that do not exist. For example...
These are all links within their internal network.

However, many of these are reachable to the outside by appending .gov to the end of the link. lheawww.gsfc.nasa doesn't exist, but lheawww.gsfc.nasa.gov does. You are already seeing ways to use this to find "hidden" domains.

The more evil-ish uses for this; however, require gaining access to the internal network. Here are some interesting peeks into some internal networks: site:apple site:ebay