Successfully complete the game to unlock the Museum bonus level. Go to the single player missions, select the new "Museum" mission at the bottom, and choose a difficulty. When the level begins, it is recommended you first study the layout. Get two weapons, and max out your ammunition through the crates supplied. Go to the desks by the entrance to the rooms. On the desks there is a red bell that is labeled "Don't Press". Once you press this button, enemies will begin coming to life in one room area at a time. About thirty people (all characters from the game) will try to kill you. You will hear a sound once that wave is completed. There are two waves of kills needed to complete it. There are three rooms, one of which nothing happens in. Do not bother shooting anyone prior to using the bell, as it will not count.