I did know that, but I'm not sure how that applies. In your scenario, the tank ruptures, there's virtually no water expansion, so nothing happens. In my scenario, the tank ruptures, the internal gas content instantly expands by (say) 250 times, creating a massive force which I assume is transmitted virtually undiminished, through the water, to the sides of the container, which promptly explode outwards. I do accept that some of the force will go upwards, but I believe the container will still explode. Not really trying to disagree, just trying to get my head around it :-)