Enter the hashbang convention, well documented by Google. The #! part is incidental; the spec is all about how site designers need to create a whole second set of statically generated HTML pages just for Googlebot, all behind URLs that include _escaped_fragment_ in their paths. There's a 1:1 mapping between the #! URLs for humans and the _escaped_fragment_ URL for robots. My Twitter page at https://twitter.com/#!/nelson, for example, also exists at http://twitter.com?_escaped_fragment_=/nelson. A special page just for bots, which Google dutifully translates to #! URLs for humans. It's all a big, gross, well-intentioned hack to work around a fundamental limitation in Google's indexing technology.