China's internet censorship is regarded by many as the most pervasive and sophisticated in the world. The system for blocking sites and articles is referred to as "The Great Firewall of China". According to a Harvard study conducted in 2002, at least 18,000 websites were blocked from within the country, and the number is believed to have been growing constantly. Banned sites include YouTube (from March 2009), Facebook (from July 2009), Google services (including Search, Google+, Maps, Docs, Drive, Sites, and Picasa), Twitter, Dropbox, Foursquare, and Flickr. Google was planning to launch a censored version of its search engine in China, blocking information about human rights, democracy, religion, and peaceful protest, but it was terminated. Certain search engine terms are blocked as well.