For those of you that enjoy the TV show Lost, what follows will make sense. For those of you that do not partake of Lost please move along, nothing to see here.
Being a fan of Michael Crichton the "smoke monster" that has appeared throughout the series is clearly a swarm of NanoBots - I can imagine no other reasonable explanation (Occam's razor applied). As the series has progressed it has been revealed that no matter how fantastic the appearance of plot devices, they have all had reasonable explanations. Supposedly Damon Lindelof, one of the creators/writers has discredited this theory. Time will tell.