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As a final point, Dragon Magazine #221, in the segment on 101 uses for a Portable Hole, says this:

Finally, the DM must consider the orientation of items within the device. Specifically, does gravity from outside of a closed hole permeate the interior, or does the interior somehow have its own local gravity?

Even an official D&D publication says that the DM needs to determine how gravity works within a Portable Hole

In short, there is no conclusive evidence at all how a Portable Hole interacts with gravity. This is entirely up to the DM to adjudicate.

In short, there is no conclusive evidence at all how a Portable Hole interacts with gravity. This is entirely up to the DM to adjudicate.

As a final point, Dragon Magazine #221, in the segment on 101 uses for a Portable Hole, says this:

Finally, the DM must consider the orientation of items within the device. Specifically, does gravity from outside of a closed hole permeate the interior, or does the interior somehow have its own local gravity?

Even an official D&D publication says that the DM needs to determine how gravity works within a Portable Hole

In short, there is no conclusive evidence at all how a Portable Hole interacts with gravity. This is entirely up to the DM to adjudicate.

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DMG185

As 5E uses plain English definitions when more explicit ones are not given...Extradimensional simply means

Looking at other things that create Extradimensional spaces doesn't clarify things either. For example, if we look at the Bag of Holding (DMG153), there is nothing in the description that specifies what happens if you upend it and shake. If you turn it inside out, everything falls out, otherwise pulling things out of it requires an Action. 

As 5E uses plain English definitions when more explicit ones are not given...Extradimensional simply means

Looking at other things that create Extradimensional spaces doesn't clarify things either. For example, if we look at the Bag of Holding, there is nothing in the description that specifies what happens if you upend it and shake. If you turn it inside out, everything falls out, otherwise pulling things out of it requires an Action.

DMG185

As 5E uses plain English definitions when more explicit ones are not given...Extradimensional simply means

Looking at other things that create Extradimensional spaces doesn't clarify things either. For example, if we look at the Bag of Holding (DMG153), there is nothing in the description that specifies what happens if you upend it and shake. If you turn it inside out, everything falls out, otherwise pulling things out of it requires an Action. 

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Ask your DM, it is never clearly specified.

Having looked at the description of Portable Holes across every edition, there is no mention of gravitational behavior to be found. The 5E description of what a portable hole creates is...

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. The cylindrical space within the hole exists on a different plane, so it can't be used to create open passages.

As 5E uses plain English definitions when more explicit ones are not given...Extradimensional simply means

Originating outside the known reality of the universe.

Source

Additionally, the term 'Hole' and 'Deep' are no help. Hole is simply a hollow space in a surface and Deep is...

extending far from some surface or area: such as

a: extending far downward

b: extending well inward from an outer surface

c: extending well back from a surface accepted as front

d: extending far laterally from the center

Source

So, Deep has multiple meanings, so we can't use this to rule on a particular sort of orientation.

This tells us absolutely nothing about how the inside of this hole behaves...let's look a bit deeper.

Looking back to D&D 3.5 we can find this definition that tells us that, technically, the extradimensional space inside of things like a Portable Hole may be considered a sort of demiplane

Demiplane: This catch-all category covers all extradimensional spaces that function like planes but have measurable size and limited access. Other kinds of planes are theoretically infinite in size, but a demiplane might be only a few hundred feet across.

A Plane of any sort, including Demiplanes can have normal gravity, light gravity, heavy gravity, no gravity, objective directional gravity, or subjective directional gravity. In short...planes can have any sort of possible gravity, or none at all.

Looking at other things that create Extradimensional spaces doesn't clarify things either. For example, if we look at the Bag of Holding, there is nothing in the description that specifies what happens if you upend it and shake. If you turn it inside out, everything falls out, otherwise pulling things out of it requires an Action.

In short, there is no conclusive evidence at all how a Portable Hole interacts with gravity. This is entirely up to the DM to adjudicate.

Options

I have generally seen this run 4 possible ways.

Portable Hole has local gravity pointing 'down.'

In this case, your wine collection is perfectly safe. The portable hole has its own local gravity and so climbing into it is always climbing down.

Some DMs I know are cautious about this ruling, because it allows players to have a deployable gravity trap that points in any direction.

Portable Hole is perpetually subject to local gravity

Under this ruling, a portable hole acts like a physical bag. The 'center' of the cloth always determines which way is 'up' regardless of whether the hole is open or not. Thus, when you pick up the Portable hole and shove it in your pocket, you just jumbled the insides up horribly.

In this case, your wine was doomed from the start.

I've found that most DMs and Players don't really like this model, because it means they can't organize their loot, or have a little shelter in the hole, or whatever.

The Portable Hole normally has no gravity, but adopts the gravity of wherever it is opened for however long it is open.

This is pretty self-explanatory, but I've never seen anyone actually be entirely happy with this (apart from brief funtimes in null-g) This is because things can potentially drift while inside the portable hole, then fall and break even if you open it in the same orientation.

Again, your wine collection is screwed.

The portable hole adopts the gravity of wherever it is open, and retains that gravity until the next time it is opened.

This is an expansion on the prior one, and probably the interpretation I've seen used the most. In short...everything in the portable hole stays where you put it, until you open it up again...when everything re-orients to whatever new position it's in.

Conclusion

In three out of four possible interpretations, your wine collection is in trouble. Might I suggest investing in clamps, glass case covers, or some other means of restraining your prized orcish wine?

But, ultimately, the rules are silent on the specifics of how gravity works in a portable hole, so this is left entirely to the judgement of any given DM.