Australia is the key to everything.. That is what Hurley said right? The first thing that I thought of is the special properties of the place where Issac the faith healer is.. Maybe that place has some of the same special properites as the Island.. remember the faith healer told Rose some people he can help and some he can't cause of the special properties there.. I figured it was some connection like that.

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Please don't give up, Des. Because all we really need to survive is one person who truly loves us. And you have her. I will wait for
you. Always. I love you, Pen

DESMOND: I love you, Penny. I've always loved you. I'm so sorry. I love you!
PENNY: I love you too.
DESMOND: I don't know where I am, but--
PENNY: I'll find you, Des--
DESMOND: --I promise--
PENNY: --no matter what--
DESMOND: --I'll come back to you--
PENNY: --I won't give up--
BOTH: I promise. I love you.