As the Mission: Impossible fuse burns, Eko ducks over by the wall with the mural, and Charlie says "bollocks" when he realizes that Eko didn't exactly set the longest fuse in the world, and he takes off running down the hall. And finally, finally we have someone being chased by a huge explosive fireball. They should do this every show! How hard could it be to come up with an excuse for that every show? Even if it's, like, Sawyer lighting farts or something!
We come back from commercial in flashback, with Desmond being awakened by the timer alarm going off. He yells Clancy's name, but there's no response, so he himself runs into the computer room and enters the numbers., and watches the timer reset. He hears some singing from behind him, and there's an open trap door or grate or something, an empty bottle of Dharma booze on the floor beside it.
He drops down into a crouch like he's a ninja into some sort of tunnel underneath the computer room. In front of him, a drunk Clancy is dangling a key from his hand, muttering that he couldn't do it. He's lying next to some sort of lidded keyhole thing in an alcove, with Caution: System Termination printed on the underside of the lid. "This is the only other way out, partner," he says, explaining to a confused Desmond that this is the station's "failsafe. Turn this key and this all goes away."
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 doesn’t care anything going on down here."