An inundation of gas occurred on MMU 001-0 Section. While advancing Panel #16 cutting Gob-I on across from #6 right crosscut at #27 break. East Mains sealed area was cut into at #27 break on panel #16. This was located approx 100' from #6 entry survey station #3325, w/last 25' unsupported. No injuries reported, temp vent.controls constructed & seal built according to plan.