The Wikipedia web site suffered an outage yesterday, caused by power problems at one of its facilities. The site was completely unreachable for 40 minutes, and experienced performance problems for much of the afternoon.

Begun in 2001, Wikipedia has rapidly grown into the largest reference website on the Internet. The online encyclopedia is a wiki, allowing Internet users to create and edit content. Wikipedia is the ninth-most popular web property, according to Netcraft’s rankings of the most visited sites. Wikipedia’s English language site is the 29th most visited URL by Netcraft toolbar users, but the higher-ranking URLs include related sites all associated with the eight top-ranked web properties – Google, Yahoo, Microsoft, the BBC, CNN, eBay, Fox News, and Amazon.com.

A dynamically updating chart of Wikipedia’s web site performance is available. Netcraft offers a web site performance monitoring service that provides similar charts, along with e-mail alerts when an outage occurs.