Well in the beginning of the show Locke says if you had told me there was C4, then he would have been more careful or something like that... I don't know how he got it but he knew about it.. He said all he did was play the silly chess game again to Sayid and Kate when he got out of the flame.. I think he was lying about playing the chess game again.. Why play again? He would not have... Not after getting to the screen with the options.. So I think he did a little looking around for himself and even possibly blew the flame up on purpose to keep the communications down... Sayid could possibly have fixed a lot of the problems with the flame... Remember how he fixed the computer in the swan when they first got into it??

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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.