I haven't seen a post about this yet but I believe we learned two important things about when the island sank this last episode. I know several believe it was the incident that caused it but I'm not so sure.
Roger, Bens father, said that they were on the island and that they should have stayed making me believe that maybe it wasnt a terrible accident that forced them to leave.... maybe they left of their own free will and not in the instance of a disaster.
Also Alex was off the island which leads me to believe her mother and father never made it to the island in 1988. However I suppose also they could have and her mom could have escaped and thats why she is now a single mom working two jobs but I believe its the former that is true.
What do you guys believe? Do you think it was the incident that caused a rift in time? Or could it have been something else?
I couldn't remember when exactly Ben made it to the island so I put 1970 to when Alexs parents originally crashed. I believe somewhere in between there something occurred to sink the island.