Christina - I like your fire backup - more fire makes me happier.

The Scuba situation is interesting. Shouldn't increasing exterior pressure make it impossible to open submerged, but once you return to the surface, assuming there was no gas or liquid exchange past the two o-rings, shouldn't the interior pressure again match the exterior pressure and shouldn't it operate normally? At high altitude though I might think the higher interior pressure might blow the cap, reclosing it would be a mistake because then returning to say sea level would mean you've got low pressure under higher pressure making it difficult to open.

If someone has one of these that seems welded shut, it isn't much more than a paper weight anyways, I vote they put steel pins through the top and bottom lanyard holes, secure each pin to the sides of a large vise (weld them on?) and slowly screw the vise open - see what happens. You may rip the pins through the brass or aluminum, you may pop the seal, or the whole thing may EXPLODE (probably not, but I like to disclaim). Doing this is unsafe, so if you decide to risk it wear protective gear! Have jacket, pants, gloves, and face/eye shield, ear protection as well. While I would do this, I have bad judgement, anyone reading this should not attempt such a silly and dangerous thing.