It is also where the Gantz Sphere is located and where Gantz's chosen dead are 'teleported' after having died. It has another room no one thinks to open until till later in the series containing more equipment. The walls and exits cannot be physically touched and there's no phone reception either. After each mission is complete the survivors are teleported back into the room completely healed of any damage they received (including lost limbs and as long as they are alive even if it's on just barely) and the front door opens, letting the survivors leave until called again. When a new mission is about to start, the survivors of the previous mission are teleported back to the room along with any newcomers.