Now that I'm a little more lucid and rested, I realize that the builder simply created it on the surface of the map and then copy pasted it at some random elevation underground, air pockets and all. Something I vehemently oppose, due to said mob rooms forming unless the area is measured perfectly and in a perfect rectangle. Now I get to check all the other caves in the region because I suspect that none were done the hard way. Nearly all were all done this way.