Impossible Concept #5: The Gelth
Shrieking ghosts with a love for possession, murder and universal domination - what's not to like? Unfortunately thanks to gases seeking equilibrium there simply can't be anything there to like in the first place. Like any other dead animal, humans go through the unpleasant process of producing gases as they decay, so the gaseous forms of the Gelth would simply have no way of entering an area which is already under high pressure. While these creatures lack any scientific plausibility compared to the fate of real-life corpses, they more than make up for that by saving us from the alternative of the only TV show possibly more gruesome than Gillian McKeith's latest poo-sifting outing. When we consider that, their huge entertainment value is simply an added bonus.