My friend and I ran into a zombie in the town of Khelm but it was partially below ground as it chased us. It was still able to hit us, but we were having difficulty hitting it,so we ran uphill out of town.

When we got up the hill a bit, we found that it and at least one other zombie we're still following us , but we're completely below ground.

Unable to kill them, we changed to another server. After a few seconds we heard another zombie closing in on us (on the new server) but could not see it. I got hit by it, but never could see it, so I assume it was underground as well.

So I'm guessing that somehow the zombies are spawning below ground in that area, at least some of the time.

I'm loving the game so far, just FYI !