4 spaces apart seems like a good middle ground. I've heard zombie noises at one point, so I know there are caves in the area, or lava pools.
I suspect that there is a ravine running along the main corridor because at one point I hit water and when I took a brief look (VERY brief, like a few seconds), it looked like a very open space. I'll return later with some jackolanterns so I can see the immediate area better (I might turn on clear water, but I don't think that will help much when the only source of light is the jackolantern anyway. Besides, its my mine and I can do what I want with it.)