Why the island was under water... The definite Answer by Chad

I have a theory about the show that I have not heard expressed before anywhere. First off since the beginning and introduction of Eloise Hawking she has served as someone who has tried to get people to where they are supposed to be. The same with Widmore and his associates in their dealings with Locke when he teleported to Tunisia. They also wanted Locke to succeed in getting everyone back to the island. On the island the Others have been following the orders of Jacob who has also served as a mysterious and hidden figure. Most of his orders have been lists and and or tasks for the Others to do without question or understanding as to why they are doing them. One example would be why they abducted certain survivors and why they were building a runway for an airplane (Ajira Airways) and didn't really know why. The reason is they were looking out for themselves as it was crucial that Desmond push the button and then ultimately crash flight 815. Reason? So the survivors could be used in the game to allude Smokey/Man in Lock for Jacob, and ultimately save the island from sinking into the ocean and saving Eloise, Widmore, and the rest of the Others. Because Jack and the gang crashed they had a reason to go back. Eloise and Widmore knew that they would end up in 1977 because they were there in '77 and interacted with them. They knew that Jack dropped the bomb and that the knowledge he gained from Daniel Faraday (which is why it was important he be there as well) into the hatch that the Dharma people were digging.

They also know that it prevented the Dharma people from sinking the island, because had Jack not been there to drop to the bomb, Radzinsky would have kept drilling and created a far worse outcome. The island then would have imploded and sank into the ocean killing everyone. This is why Eloise was so worried and worked so hard to get people into place and make sure it still happened that Jack would crash, Jack would go back, and Jack would create the incident.

NOW... if Jack's dropping of the bomb down the hole prevented the incident, then Oceanic 815 would have never crashed correct? YES.

Now if the PLANE NEVER CRASHES, then Jack NEVER GOES BACK to the island either. This would create a new timeline where everyone on the island would be at the mercy of whatever the Dharma people were doing. In this case drill into a massive pocket of energy sinking the island creating this alternate universe we are seeing now. In this universe, Eloise and Hawking are D-E-A-D. So you can see their motivation for getting everyone back to the island. Jacob, Smokey, Eloise, Widmore and the rest of the Others life depends on it. Tada... Mystery solved!