This is one major and very big easter egg, I am not the one who found it though!

First open all the areas in Shi no Numa and activate all the radios, having a crawler will help. When you do that a deep voice will call out random numbers. Now make sure you have juggernog and then walk through the traps while they are active, one at a time.

Then one of them will teleport you to Der Riese and it will open the whole map of Der Riese. You will also get your pack a punch your weapons that you had in Shi no Numa.

This is a very fun easter egg to do. I am not sure if when you get the pack a punched weapons they come with full ammo and I also don't know if this works on multiplayer.