(adsbygoogle = window.adsbygoogle || []).push({}); I don't know if anyone has thought of this. During the trailer you can see Zombies get faster as they move on to the next area. So my guess is they show the level of difficulty for each area obviously the skulls are the hardest difficulty. Maybe you need to get certain amount of kills then the bus takes you to the next area of difficulty until you can't survive anymore.