Even though you may see http://www.google.com/ in Internet Explorer's address bar, it is really http://www.google.com/index.html on the server. Index.html is the only file name invisible. However, some pages have an index2.html that you can see in an address bar.
When someone stumbles upon Urban Dictionary when looking for words on Google, thinking that Urban is the name of just another dictionary publisher. They look up a word, find something silly or offensive, and wonder what Urban Dictionary is.
I got Urban Dictionary syndrome in 2007 when I discovered Urban Dictionary looking for big number names like googoldecaplex. I looked up another word and was shocked at a word that came into view.