Main Ballast Tank 6A is outside the pressure hull wrapped around the Forward Engine Room.
Inside Main Ballast Tank 6A you can see the smaller round pressure hull with the external frames. Also visible are high pressure air bottles that are part of an air bank. The pipe that leads up from the pressure hull to the outer hull is the sanitary tank discharge. Looking up you can see the ballast vent valve used to let air out when filling the tank, and the open manhole (normally sealed) used by the photographer to get into the tank.
Another view of the ballast tank vent valve.