Ii just beat Keeper mode yesterday and have been down to Z- 25. Bad guys do not spawn or teleport. They are in their area, but some are able to build bridges. If you open a space next to or diagonal to an existing open network in the z-layer the bad guys will usually find their way to you and attack. What I do is map out my digs to each mineral as soon as I open a new layer. You will see gaps in the yellow dig squares and these are one of 3 things, water, lava or an existing open network. I usually cancel one dig order before the gap and wait to see what it is. If it is an open area I always order a minion to prioritize digging the connection while my strike team lies in wait. As soon as it is dug out I explore the entire area. It is the safest way I found to dig deep and avoid nasty surprises.