Let’s say you place a Portable Hole against a wall, then walk inside its hole. Then, an evil wizard casts a fireball in the room where you put the hole.
Considering you are now in an extra-dimensional place, are you out of range of the fireball?
(The question also applies to an archer that would try to shoot at you from outside the hole.)
The description of the portable hole says:
A breathing creature within a closed portable hole can survive for up to 10 minutes, after which time it begins to suffocate.
which implies that if the hole is open, air can flow in and out. And if air can get in and out, then so can fireballs and arrows.
Furthermore, the description says:
Any creature inside an open portable hole can exit the hole by climbing out of it.
It does not specify any special restrictions on climbing out of the hole, so such exiting must just be subject to normal movement rules. It doesn't even bother to provide any description of entering the open hole, so there's every reason to believe an open hole is just an open space, and a fireball will surely fill an open space within range, and there's nothing in the text to suggest a barrier to arrows.
some reflections 5 years later (February 2024)
Nobody's answer highlights a very valid point. The portable hole can be interpreted to allow shenanigans. Game-breaking shenanigans.
If you are the DM, it's your job to make the game fun. The rules are not in charge. You are in charge. It's your role to make the game fun. When you think game-breaking shenanigans are going to break the game, shut them down. If you need to justify this to yourself, see the Introduction to the Basic Rules, the Introduction to the Dungeon Master's Guide, and the Introduction to Xanathar's.
Every game I've played in, as player or DM, the DM has been quick to shut down game-breaking shenanigans, and as a result, the games move more quickly, and are more fun for everyone.
In this particular case, if you feel that the portable hole is going to be exploited, as an example, tell your players, "I'm glad you brought that up. 5e is a great ruleset, but there are many inconsistencies. This is one of them. In this setting, the portable hole works like rope trick, (see Gael L's comment) in that "attacks and spells can't cross through the entrance into or out of the extradimensional space". Or, if you want, let them do it once and have it blow up the hole, if allowing it to work would be more fun. For that matter, let the bad guys do it too, if playing "portable hole wars" seems fun.
Or perhaps, if your players are smart enough to test the edges in world, and research it in game, tell them something like, "Roll an arcana check. Hmm. Well, you've heard stories of Ziphylberd the Singed, who experimented with similar tactics, and blew up the hole and half the town. It's said their partner, Berdylziph the Slightly Less Singed, remarked afterward, 'you see, this is why we can't have nice things.'"
Either way, or whatever, don't waste time or lose sleep over adjudicating the rules to make the game fun. That's the DM's job.
Bravo for you for looking for fun exploits.
Your best bet is not to surprise the DM, but to discuss your plans ahead of time. A smart DM is on the lookout for this sort of thing. A smart DM is thinking, what are they up to, not to spoil your fun, but to make the game fun for everyone. A smart DM may limit the knowledge given away for free, but even if so, your DM knows what you want to try.
The best exploit won't be game-breaking, but in cooperation with the DM will be something that gives you something fun that makes the game a bit more exciting for everyone.
The Portable Hole states:
It unfolds into a circular sheet 6 feet in diameter. You can use an action to unfold a portable hole and place it on or against a solid surface, whereupon the portable hole creates an extradimensional hole 10 feet deep.
This suggests that once unfolded, there is now an open entrance that is 6' wide on the outside and 10' deep on the inside.
A hole that is within the range of a fireball would definitely put anyone/anything inside at risk because Fireball states:
The fire spreads around corners. It ignites flammable objects in the area that aren't being worn or carried.
The magical fireball would extend into the hole and ignite anyone/anything inside it.
There is also a potential risk that the Portable Hole could be damaged by the fire as well.
This is more of a DM decision, because it is unclear whether or not you can see into the environs of the portable hole. I could see a case for either, but it will depend on the GM determine line of sight. If there is one, you can fire at a target, if there isn't, then you'd have to pick a location in the hole at fire at disadvantage (unseen target.)
The entrance to the portable hole is not a normal, physical opening. It is a gate or portal to an extradimensional space, that works to transport things from where you are into or out of the hole.
While the game does not define how distance works accross portals in general because portals are so varied, the opening to a portable hole seems to be very similar to a physical opening where distance works normally:
If you can drop an object in, you can also shoot an arrow in. Both are objects.
For a spell like fireball, the hole has no language against counting distance normally or about blocking effects. Compared this to rope trick, that says "Attacks and spells can't cross through the entrance into or out of the extradimensional space". You would expect similar language, if it was not possible.
So, it should work and it seems likely that effects could extend into and out of the hole, and you can cast spells into and out of it, using normal, linear distance.
Allowing this for spells however enables shenanigans with glyph of warding put onto the walls of the hole, that can kill pretty much any opponent. So the DM may decide that for balance reasons this won't work. Before you rely on it, check with them, as they have the right and a valid reason to rule otherwise.
