how you unlock the undertown?
Go streaking in Border Town until your rep gets to -5 or something like that. You'll get sent to the undertown for punishment. After that, you can always get to Under City, but you can't get back. I think exploring the Under City to 100% unlocks the path? But I had to reload the page because the game broke, and now the path back to Border Town is gone and I can't seem to get it back.