I have an infection, but it is not to the last level yet. Is that why I can't dig?

I'm pretty sure, considering you can't get into Skitter's until level 6... I would think it would apply to the Underground as well.

I'm facing the same thing. :/

Both the Underground and the Zombie Den are for zombies only. Even though there are 8 levels for each infection this year, you are considered a zombie when you reach Level 6.

Bah! Thank you, though. I just wanna hurry my progression up! >;(