In Dungeons and Dragons, we called this a bag of holding. Had something to do with dimensional warping.

I recall a discussion about a suitcase built such that you could access the 4th dimension within it. A guy would put various items into the suitcase, then remove them, one at a time, before placing the next item in the suitcase, circulating about 10 times the volume of items than it would hold fully loaded. Later, he would open the suitcase and remove whatever item he needed as it appeared.

Kinda along the same principle as the clown car in the Circus I suppose. How else they gonna get all them clowns in that little itty bitty car except via another dimension? Or is it that clowns are compressible?