"This project is both very large and fairly old in Internet terms. At the time it was instigated (1996), it was not clear that web sites [and the documents made available there] would often turn out to be transient. As a result there is a process called "link rot" - which means that a "broken link" is a result of someone having taken down a web page. In some cases some websites have simply reorganized sub-directories without creating forwarding links. Since 2000, very few links to external sites have been made. An effort is under way to remove bad links."