Desmond asks what's behind the wall. "What was the 'incident'?" Oh, nothing, just geologically unique electromagnetism, stage-whispers Clancy: "The incident -- there was a leak. So now the charge builds up, and every time we push the button it discharges it before it gets too big." Desmond wants to know why they have to push the button at all, if the whole thing could have just been destroyed...I'm guessing that's the question he's asking, because Clancy interrupts him to laugh, and close the lid and say, "Here's the real question, Desmundo. Do you have the courage to take your finger out of the dam and blow the whole thing up, instead?" No, I think Desmond asked the real question, and there were probably others, like why not automate the system? Why leave it up to a couple of people, with no support? Especially people who are likely to go psycho bugnuts as they all apparently do?
Anyway, speaking of blowing things up, present-day Desmond is listening at the blast door, and tells Locke he thinks Locke's friends blew themselves up. "They're not my friends," says Locke. Cold, brother. Real cold. Desmond can't hear anything out there, so he wants to open the doors, but Locke says that could be a trick. Des says they could be hurt and asks Locke where the doctor is. Locke's all "God knows where" and anyway Jack "sure as hell doesnt care anything going on down here."
And now is the time for the pseudo-philosophical discussion between a guy named Locke and a guy named Hume -- like we GET it, you writers are SMART PEOPLE, and you prove it with the names you give your characters. Desmond asks if the reason Locke is letting the timer run down is because he needs to look down the barrel of a gun to find out what he really believes, WHATEVER THAT MEANS. Locke says he already looked down the barrel of a gun, and he believed, and he thought it was his destiny to get in here, and Boone died because he was stupid enough to believe in Locke too, and then Locke had a big hissyfit on top of the hatch, and the light came on, and he thought it was a sign, but it was probably just Desmond going to the bathroom. Well, a religion based on a foundation of Desmond taking a dump probably wouldn't be the weirdest thing. Meanwhile, Desmond's looking stunned, like something Locke has said has twigged something in him.