Who said anything about me being underground?
AFAIK in most fixed static worlds the designers of the game actually add specific routes to the game where NPCs can walk. There is no dynamic path finding at all, the basic paths are determined before you even start the game. All the zombie needs to do is select such a path and he is on autopilot for quite a while.
In a dynamic world you have to construct for ANY single block where a zombie could stand whether he can cross over to the next blocks in 8 directions. Including checking if going up or down some maximum height makes crossing over possible. Including checking if the free height is enough for standing in the next blocks.
Then you have to construct paths and shortest paths out of this huge matrix of connections and non-connections.
If all you want is the zombie just going the directest path possible then a lot of this is not needed. But then zombies would always hit the wall in front of them even if there is a large opening one block to their right. They would look very artificial because you just moving around would be exactly mirrored by their movement, they could not even go round a simple stone. Don't even think A16, their movement there was much more complicated than a direct beeline.
I don't want that, TFP obviously didn't want that either.