After completing the game, go to the single player missions and choose the bottom new mission called "Museum". When you start the level, be sure to study the layout. Get two weapons and top off the ammo with the crates supplied, then go to the desks by the entrance to the rooms. Note: There are three rooms, one of which nothing happens. On the desks there is a red bell that is labeled "Don't Press". Once you press this button, the entire museum comes to life in one room area at a time. About thirty people (all characters from the game) will try to kill you. Select a difficulty at the start. You will hear a sound once that wave is done. There are two waves of kills needed to complete it. Do not shoot anyone prior to using the bell as it will not count.