What would happen if you opened a Bag of Holding underwater?

Question

I'm in a naval campaign where some of the game could take place underwater, and I could reasonably acquire a Bag of Holding soon, so it would be nice to know before I try to grab an item underwater and jettison all of my gear into the Astral plane. Would it fill with water until the bag was filled to capacity and scatter all the contents inside as well?

Also assuming you didn't break the Bag of Holding by overfilling it to capacity, would putting water inside it get other objects inside wet, potentially ruining them?

Answer 1

It would quickly* rupture.

Five-hundred pounds of water is only about 8 cubic feet in volume, so the bag's weight capacity would be overloaded after merely an eighth of its volume capacity was reached — assuming it started empty, as any other items in it would help it reach its weight limit earlier. It would then rupture, scattering its (damp or soaked) contents across the Astral sea.

An objection might be raised: doesn't the bag only store things specifically put into it? No, the owner is not so in control. There's no verbiage limiting access to only intentionally stored items, so an open bag allows anything to pass into it. This means that opening it underwater would result in the water pouring in.

The space inside the bag is described as a continuous finite volume, not a set of pocket-spaces for each individual item put in it; therefore, putting/letting water into it would get water on any other items sitting inside.

_{* How quickly depends on the local water pressure, but just “quickly” is plenty of precision for our purposes.}

Answer 2

Nothing in the rules says the bag would keep the water out, so water would probably flood the bag.

On the other hand, nothing in the rules says that the air pressure in the bag is always 1 atmosphere. It is entirely possible that the inside of the bag magically matches the pressure of its surroundings.

If you want to be "nice", you could rule that the players are allowed to open the bag upside down (essentially turning it into a diver's bell) and blindly fish around for their items.

Answer 3

There is no fundamental difference between it filing with water and exploding and filling with air and exploding.

It is absolutely clear that the whole point of a bag of holding is that normal physics don't apply and that it's a convenient in-game way to be able to carry lots of stuff.

Having said that the idea that the magic mag treat water as different from air is not utterly unreasonable but then again if you are magically breathing underwater in the first place....

Probably the sensible thing is to have the DM adjudicate whether a) a normal bag of holding will work in the world they have devised b) whether a special underwater bag of holding is possible c) they may want you to test it in-game for yourself

Answer 4

In-game, ask the Bag's vendor to answer, and to prove their answer with a test. That way the GM can answer with their own 'physicks'.

Answer 5

When you are underwater its best to treat the water itself like air when it comes to taking up space. It will fill the bag when you open it, but it doesn't affect its capacity until you cross over onto land again where you'll have to dump the water out. Water wouldn't rupture the bag any more than air would. Water does not continually force itself in until it bursts, it simply fills the bag as it would fill any container. I might have the player who opens it underwater for the first time without being careful make a save to avoid being pulled inside by the sudden suction of the water rushing in, but that's about it. I would also say the items inside would get wet. It is one pocket plane so its not compartmentalized like a handy haversack would be. Anything like paper would get ruined instantly. Magic items and things in sealed containers would be fine.