At the "Imports" screen of Disc 1, press Square, Circle, Square, Triangle(2).
At the "Code Archive" screen of Disc 1, press Circle + Triangle, Square(2).
At the "Code Archive" screen of Disc 1, press Square + Triangle, Circle(4).
At the main menu of Disc 2, press Triangle(2), Circle, Triangle(2).
At the main menu of Disc 2, press Circle(2), Triangle, Circle(2).
At the main menu of Disc 2, quickly press Circle, Square, Circle, Square, Circle, Square.
If you line up the first four volumes of the Underground series CD packages spines, you will see the following code: Press Circle, Triangle, Square, Triangle(2), Circle(3) to reveal a secret URL (http://www.playstation.com/hiddenprize), username (underground), and password (insider). The hidden prize...well go there and find out!