Lost!

[BarTopDancer]

Originally Posted by Ghoulish Delight

I think the only answer that can be given right now is that we have no idea. Since the "flash sideways" story began, we've been kept in the dark about what is happening in the "real" timeline, whether that timeline even exists anymore, whether it ever existed. It's a complete unknown, they've given no clues in the show that would make any one answer more likely than another.

Add to that, at the start of the season the island was underwater.

So, we have

An island that is now underwater after an H-bomb was set off (presuming it actually went off and they didn't 'jump' right before it exploded).

People who have been living on the [now] underwater island still living on the above ground island and somewhere else.