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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?

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    \$\begingroup\$ I'm interested how the answer changes if you open it upside down without letting things fall out. \$\endgroup\$ – Tanath May 5 '16 at 16:48
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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.

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    \$\begingroup\$ Anyone who wants to debate how physics applies to D&D magic items beyond simple encumbrance, good luck and please use this chat room for the purpose. Anyone who has suggestions for how this answer can be improved, feel free to use comments for that. Thanks! \$\endgroup\$ – SevenSidedDie Apr 30 '16 at 0:39
  • \$\begingroup\$ @tzxAzrael See the previous comment, starting “Anyone who…” If you instead meant to suggest an improvement, you left the suggestion part out of your comment and I request you try again more directly. :) \$\endgroup\$ – SevenSidedDie Jul 19 '16 at 14:13
  • \$\begingroup\$ 8 cubic feet of water while underwater weigh virtually nothing, weight is not the same as mass. just don't leave the water with the bag foll of water. \$\endgroup\$ – John Feb 24 at 15:24
  • \$\begingroup\$ @John If the pressure of the outside water were pressing on and equalising the internal pressure of the bag (creating neutral buoyancy), but that’s not what’s happening. The extradimensional space that is the “inside” of the bag is another plane of existence—a pocket dimension—and is not itself underwater. The weight capacity of the bag’s extradimensional space is unaffected by where its portal-item is, and the weight limit will still be exceeded. \$\endgroup\$ – SevenSidedDie Feb 24 at 17:21
  • \$\begingroup\$ @John The way the item-dimension combination works regarding weight is explicitly defined by its having a weight limit. See also the first comment under this answer. \$\endgroup\$ – SevenSidedDie Feb 24 at 18:11
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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.

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    \$\begingroup\$ If you turn it upside down, won't all the items fall out? \$\endgroup\$ – immibis Apr 30 '16 at 4:13
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    \$\begingroup\$ @immibis Quite possibly. The character would have to make a small opening and hold it closed around their hand. Not an easy task, for sure. \$\endgroup\$ – acbabis Apr 30 '16 at 4:29
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    \$\begingroup\$ @Shem I think that was true in older editions, but I don't see anything like that in the 5e listing. \$\endgroup\$ – acbabis Apr 30 '16 at 6:36
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    \$\begingroup\$ @immibis The description of the item covers that: no, its contents only fall out if you turn the bag inside out. \$\endgroup\$ – SevenSidedDie Apr 30 '16 at 8:15
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    \$\begingroup\$ @Shem I think that's the handy haversack you're thinking of. \$\endgroup\$ – zwol Apr 30 '16 at 22:03
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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

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    \$\begingroup\$ This answer essentially boils down to "your dm should do whatever they want"; which is sometimes an appropriate answer, but this answer seems to be trying to pretend that's not what it's saying. It doesn't make any appeal to any rules, tries to make some kind of argument from a physics standpoint but then says physics doesn't apply. There IS a fundamental difference between filling it with air and with water (the weight), which is a relevant difference to both physics and the rules. \$\endgroup\$ – Shufflepants May 16 '16 at 18:08
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    \$\begingroup\$ I believe that the water/air equivalence was referring to the fact that opening the bag (allowing it to fill with ambient air) doesn't overfill the bag, causing it to rupture. With that in mind, why would allowing the bag to fill with ambient water cause it to rupture? Would opening it during a dust storm cause it to rupture as it over-filled with dust/sand? Or does it take an active attempt to put more into the bag than it can handle to cause it to rupture? \$\endgroup\$ – Theo Brinkman May 18 '16 at 3:21
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    \$\begingroup\$ The issue with water is that it is heavy. If you fill it with air or dusty wind, it's still under the weight limit. If you fill it with water, it hits the weight limit pretty quickly, and it's the weight limit that causes it to rupture. "do you hit the weight limit or not" is a pretty fundamental difference for something with a hard-coded weight limit. \$\endgroup\$ – Ben Barden Aug 24 '17 at 18:33
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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'.

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    \$\begingroup\$ Even uncommon magic items are worth over 100gp. I'm not sure the shopkeeper would want to risk destroying one. \$\endgroup\$ – acbabis May 22 '16 at 22:24
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    \$\begingroup\$ Could be the start of a quest to find a conman selling "waterproof" bags of holding. \$\endgroup\$ – Cees Timmerman Nov 3 '16 at 12:48

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