I should clarify something: You can build an underground base and provide multiple paths for zombies to get to you without digging. But, so far, I've found that they will STILL dig, even if there's an easier path. That's the difference in contrast to a tall base. With a tall base, or even just a base on top of a building, if you provide even just one path up to you, most zombies will take it. Some might beat on the building (especially if you get them in rage mode). But short of making the building fall, they don't have any way to get to you other than the paths you provide. And I've never experienced zombies collapsing a building unless it's a wood building or a poorly designed base like one on columns. This is in contrast to digging, where zombies will get to you one way or another no matter how many paths you provide.
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I should clarify something: You can build an underground base and provide multiple paths for zombies to get to you without digging. But, so far, I've found that they will STILL dig, even if there's an easier path. That's the difference in contrast to a tall base. With a tall base, or even just a base on top of a building, if you provide even just one path up to you, most zombies will take it. Some might beat on the building (especially if you get them in rage mode). But short of making the building fall, they don't have any way to get to you other than the paths you provide. And I've never experienced zombies collapsing a building unless it's a wood structure - which is a poor choice. This is in contrast to digging, where zombies will get to you one way or another no matter how many paths you provide.