Rockopolis is a destroyed and hidden settlement in Fallout 3. In the GNR (Galaxy News Radio) transmissions, there is a space dedicated to Herbert Dashwood's adventures calles "The Adventures of Herbert 'Daring' Dashwood", where Rockopolis is mentioned. According to Herbert Dashwood himself, everything said in the radio is true, thus making Rockopolis a real place.
You can find it north of Girdershade, west of Smith Casey's Garage and southeast of the Yao Guai Tunnels. You can find it easily because you will see pennants hanging over a boulder.
Actually, if you listen to "The Adventures of Herbert 'Daring' Dashwood", you will find out that the entrance to Rockopolis is hidden, and it is. The entrance is the boulder itself, which you can interact with as a regular door.
Also you will find out that there was an event where the raiders were led to Rockopolis and played havoc, thus reducing the place to a merely cave instead of a setlement.