Robs got it spot on, if a cylinder is going to go it will go at the weakest point - which is the O ring, it will hiss like a battered snake and louder than you can whistle, you may even pee a little, but you will be fine!
The chances of one actual exploding are hundreds of thousands to one, I would think that the whole of this forum, with it's thousands of years of combined air gun experience covering probably hundreds of thousands of guns - no one has actually see a cylinder explode unless it was a controlled experiment. (and I'm not counting seals blowing as above which is scary but fairy harmless)
On the dive cylinder bit - I used to dive for around 14 years very frequently and I've seen tanks get dropped 20ft down a pier wall onto Concreat and not be damaged. I've also seen others doing the same and the valve being damaged and guess what - massive O ring failure which was as loud as a firework - people were literal feet away and cacked themselves, but no one got hurt.
Yep - P1000 is the scary bugger
James