Why the Island is Under Water by Yetilocke

The Flash Sideways is NOT the outcome of detonating Jughead. Jughead merely cause the Incident that has always happened, which led to the Numbers having to be entered into the Swan Computer, which led to flight 815 crashing. They caused their own crash.

The Flash Sideways is a result of something else that will happen toward the end of Season 6. I believe it will have something to do with the Volcano that is supposedly on the Island that we have never actually seen. The writers of the show have always stated that Jules Verne's "The Mysterious Island" is one of the main influences of the show. At the end of this book, a volcano that was thought to be dormant becomes active again, and the magma comes into contact with an underground lake. This causes a massive explosion which pretty much completely destroys the island except for one tiny patch of land, which most of the characters miraculously survive on. I don't think that an atomic explosion could sink an island. The only force I'm aware of that has that magnitude of power is a volcanic eruption. Maybe a Hydrogen Bomb is powerful enough, but we have to remember, the Hydrogen Bomb wasn't detonated, only the nuclear detonator from the Hydrogen Bomb. Which also means that ther! e is still a Hydrogen bomb on the Island.

Then we have to understand what a 'sinking island' might allude to. There is the story of Atlantis, in which an island or continent sinks into the ocean after one day and night of disaster. Combine this with Jacob saying 'But it only ends once. And everything before that... is just progress'. Maybe the events that are happening on the Island have happened many times at different points in history. It always begins with strangers coming to the island, and always ends with the island sinking into the ocean. The alternate timeline is the result of a previous iteration of this loop, where the strangers failed to prevent the island from being destroyed. But this time our group, the 815 survivors, will succeed. That is how Jacob will prove his nemesis wrong, and it will only end once.