The theoretical plane of existence below the quantum level in Star Trek. According to that source, subspace occupies the same time and space as conventional space, but with a separate set of physical laws; even though in glimpses of that reality, it was shown to have a compatible set of laws to our own (the subspace creatures that stolen various crew members for experimentation – including Riker who’s arm was surgically removed and reattached).

I personally believe that this is non-plausible because of the vast difference between realities with different governing laws. No parallel could be drawn between that plane and ours (due to the different of basic laws).