The door ceases to exist when shut, so probably the gaseous form escape wouldn't work. We could try breaking through the stone walls of the chamber, but without knowing what is on the other side, I am loathe to do that - not knowing the mechanism is crippling, because I don't want one of our experiments to suddenly make it stop functioning. We know it's a conjuration effect, but that's it; Identify gave us no answers. (However, when the door is closed, the area where it should appear does not give off any magical aura, which is why I believe it might be a [Teleport] effect). I believe the item may be Plot Central, which would explain the enormously out of scale value.
Of course, waiting a couple levels until we have access to Teleport would be the easiest way to determine that, but we would like to begin using this as a safe base of operations as soon as possible, since obviously that would be a powerful effect.
The solutions of leaving a party member outside, or just leaving the door standing open, strike me as unsatisfying. That's what we'll have to end up doing if nothing else presents itself, but the prospect of having the door closed while we are sleeping (say, if the party member was stealthily attacked or dominated) is not a pleasant one. Something like leaving the open door against a surface and concealing it with a silent image is a little better, but if something so much as accidentally knocks against it and closes it we are fucked in the portable hole, as it were.
So I guess more specifically what I am asking is: Is there a safe way to leave the door open with all of us inside it, or is there a surefire way within our means to tell what kind of effect this is so we can figure out some way of leaving the door closed and still being able to leave easily (and if so how do we protect the entrance properly, since it can be opened again from the outside).
Thanks for the help so far!