I just have a random theory about what the Event is. I was watching the latest episode the other day and a thought occurred to me. Maybe the detainees are people from an alternate universe (or reality if you prefer) a la Star Trek's Mirror Universe. These people obviously aren't exact copies of anyone from our side as proven by their longevity. This is mainly a theory based on no evidence, but the second "E" in the opening title is reversed which makes me think of a mirror. The Event in question could be their emergence into our universe/reality.

What do you think? Feel free to poke holes in this, just passing idle thoughts during my TV watching.