What, pray tell, will be the effect of air pressure that I'm ignoring? And why shouldn't we ignore it?
The effect, plausibly, would be expansion of the bag interior until the pressure inside the bag is equalized with whatever pressure is outside the bag in this nonspace, flowing from your spandex analogy. That could, with sufficient variance, cause the bag to exceed its volume limit, as I think on it.
Why shouldn't we ignore it? I think we SHOULD ignore it. Having the Bag explode due to air pressure, or air weight, adds nothing positive to the game. But I also question why we should get hung up on the idea that water fills up the bag and explodes it. That doesn't seem to add anything of value to the game either, so I question why we shouldn't ignore that as well. Or, more to the point, why ignoring that as well is perceived as so horribly wrong that it should never be countenanced, but rather dismissed with your "paisley dragon" assessment.
The bag holds 10 minutes worth of air. It does not fill to its volume with air. Why should it fill to its volume with any other medium? When physics oppose magic, magic wins. You interpret the bag to expand and morph its interior dimensions. Where in the rules is this written? I don't think your "not explicitly in the rules" assumptions enjoy any greater validity than anyone else's.
You insist on referring to "what a normal container" or "what any other bag" would do. A Bag of Holding is a magical item, not "a normal container" or "any other bag". I posed the question at a game over the weekend. The off the cuff response was "well, the bag isn't really a bag - it's an aperture to an extradimensional space. I'd rule that objects must be passed through that aperture." That was with the caveat that he'd accept a lot of other rulings, but his feeling was that this was most consistent with (his vision of) the magic of the bag.
Or more to the point, why should it be such a big deal that it's being dragged out this way? The difference between 20 gp weight on average at sea level an 18 gp weight at altitude? Or was he worried about the fractions of a gp weight variance that comes from weather fronts?
It's not a big deal, which is why I would not track it in my game. But I don't think it's a big deal to rule that water will not flood and destroy the bag because the magic of the bag in some way prevents it. Another possibility my other commentator mentioned was that the bag would flood to the same extent air would have filled it, water damaging the objects inside. That was the "other ruling he would accept" that sprang to mind. When I suggested that water would flood the bag to exceed its weight capacity, he got a very puzzled look and dismissed that as a ruling he would not have considered, and one that seemed to ignore the fact that the bag
isn't just a sack like any mundane one, but a magical item that allows objects to cross dimensions. So why would it act like a normal bag in that regard?
Or was he perhaps worried that air pressure would over inflate the bag, like a balloon, and ensure that the item can't actually exist at all?
It can't exist at all. Unless we accept, as we do for the game, the existence of magic. Having accepted the existence of magic, which permits an impossibility, we have moved beyond requiring the application of the rules of physics to every aspect of the object's existence.
I believe his point was that if the bag doesn't behave predictably and rationally in respect to air content and pressure, there's no basis for assuming that it will do so in respect to water pressure.
Bingo.
In fact, since the bag is noted as containing enough air to sustain a creature for 10 minutes, regardless of its size or other current contents, it would be quite valid to conclude that it retains that air content even when immersed in water.
Quite true - hadn't thought of that possibility. Another suggested at our discussion was that it contains no air - it contains some medium in the extradimensional space capable of supporting an airbreather for 10 minutes, which replenishes when the extradimensional portal is opened. Again, it's magic and another dimension.
The argument seems to be that water entering the bag will behave irrationally and/or unpredictably because there are specific rules for air that touch peripherally on the cubic footage, and thus implicitly say that the bag won't inflate and burst due to air pressure.
I know I said I'd try to be nice, and I am, but I honestly can't think of way to sum up that argument that doesn't look like mockery.
Ignoring the "10 minutes of air" rule, and the subsequent calculations on volume and weight for a moment, why would air be a problem. The plane the bag bursts into (Astral) isn't an airless void. So the bag isn't a stopper between us and absolute vacuum, and won't inflate from air pressure any more than any other bag would.
In short, the bag behaving as it does, with regard to air pressure, is entirely rational and predictable, even without the hard and fast rule limiting the air space inside.
Air isn't a problem. But it's also explicitly ruled to not behave rationally or consistently. Regardless of the volume of the bag, the air lasts for precisely 10 minutes. Regardless of whether the living creature in the Bag is a sleeping mouse or a rampaging elephant, the air sustains it for precisely 10 minutes - that cannot be the same volume of air. Regardless of whether the Bag is empty or nearly full, other than the creature, the bag maintains exactly the same 10 minutes worth of air. It defies the laws of physics. And that's fine - that's what magic does.
But you are insisting absolutely that it is ludicrous to even consider any ruling that the bag might, because it is magic, defy the laws of physics in any other way. Do you also rule that the PC's suffocate after casting Fireball underground, since the huge flame consumes oxygen? That is what fire does, as you doubtless know. The rules do not say this magical fire fails to consume oxygen, do they? Well then, they must not change that rule of physics, and every adventuring party that has ever cast a fireball underground should be long since dead. Or is part of the magic that the fire somehow does not use up all the oxygen in this confined space?
We accept an awful lot that defies the laws of physics in a fantasy game featuring magic. Why is it so essential that water flood a Bag of Holding for the game to remain playable? I think plenty of alternative rulings backed up with at least a plausible "magic works like this" explanation have been provided.
Equally, water pouring into an unsealed container when it's immersed is also completely rational and predictable. So is the idea of bag tearing if overstuffed or overloaded.
Once we accept that the Bag allows access to an extradimensional space which enables it to hold more volume than its own capacity and never change in weight, I think we have bypassed the idea that the Bag is "completely rational and predictable". Your insistence that there is only One True Way to interpret the functioning of that magic, and the manner in which it would interact with immersion, cannot, in my mind, be irrefutably supported by physics once we have accepted that physics have been overridden.
What's not rational (though at this point very predictable) is the argument that "The rules don't say it doesn't have an automatic safeguard against a condition that the authors probably never considered, so there must be one."
Who has said there
must be one? I have said your ruling is valid, but not the only valid ruling. I don't recall anyone else saying that your ruling was invalid, just that it is not the One True Way. I think the case that the flooding of the bag (or, for that matter, its explosion due to air pressure) would be a flaw casters would work to fix had considerable merit as well.
I doubt, however, that I'll buy into the air pressure argument in any of its permutations.
Nor will I buy into the argument that magical items must conform to the laws of physics, or must do so in all instances where the rules do not specifically indicate they deviate. As you have alluded to, the rules cannot cover every possible permutation and combination, so that leaves us to make reasonable rulings. Rulings which add to, or at least do not detract from, the fun of the game. Rulings which may be inconsistent with the laws of physics, especially where the existence of magic may suggest or support it.
The fact the rules do not say the Bag's interior dimensions vary, or that the bag interior is stretchy, does not mean your ruling is the equivalent to paisley dragons, but neither do I perceive the other rulings posited on this thread to merit similar dismissal.
