RockBolt Inside the tank was superheated air, and the tank was left sealed as it cooled to ambient air temperature. As pressure * volume = temperature * gas constant, a constant volume and substantially lower temp = proportionally lower pressure. That pressure inside gets low enough that atmospheric pressure outside is enough of a differential to crush anything like a beer can
Spastic Avenger I think its also because the tank is designed to have pressure acting outwardly and not inwardly.
Jefka You can see a hose attached, so I'm not sure it's just Joule-Thompson effect cooling.