Rockopolis

Rockopolis - This location is not a major landmark nor a random location and will not appear on your map. It is located due west of Smith Casey's Garage, due north of Girdershade and South East of the Yao Guai Tunnels. The entrance is a boulder that can be interacted with like any other door or entrance. Facing the boulder, there is one dead tree on the right, two on the left. And as the attached pictures show, there is a string of flags above the boulder. It is probably easiest to find at night, as there are lights glowing on the rock face as you approach it from the East. It should also be noted that the entrance is very close to the center of that particular square on the map.

History
According to files on Herbert Dashwood's computer and the accounts of The Adventures of Herbert "Daring" Dashwood on Galaxy News Radio, Rockopolis was once a hidden underground village. The inhabitants would only allow outsiders in if they presented with a secret knock. The leader of Rockopolis at the time was King Crag. Herbert led Penelope Chase, a slaver leader, to Rockopolis. According to Herbert Dashwood's computer, the incident led to the enslavement of the city's population and the destruction of the city itself. No new residents, human or otherwise, have taken up residence since.

DLC Connection
Inside Rockopolis, or what can be accessed, there is a note from a slaver talking about the slaves in Rockopolis, and there next location: The Pitt. It is supposed the bouldered off passage leads to the rest of Rockopolis, and supposedly the Pitt. This the Highest speculation, as the Note from the Slaver shows the biggest hint by mentioning thee Pitt. In 2009, this will finally be answered.