Go read this; the following excerpt sets the scene:
These [tanks] are usually equipped with pressure relief fittings, since nitrogen does tend to want to be a gas, and gases do tend to want to expand quite a bit. This tank, though, which seems to have been kicking around since 1980, had been retrofitted by a real buckaroo. Both the pressure relief and rupture disks had failed for some reason in the past, so they’d been removed and sealed off with metal plugs. You may commence shivering now.