Let's see. First, people with pacemakers start dropping dead, all at once. Then the pigeons of Trafalgar Square lose their innate direction-finders and fall plop on everybody. Then the shuttle Endeavor, coming in for a landing, finds itself about to pancake on Dodger Stadium because its navigational system went - and here I must use the technical term - kerflooey.
So what's going on? Well, if you must know, it's just the earth's core. Somehow it stopped spinning and generating electromagnetic waves that protect us, you and me, from death by radiation. We'll all die. Let me rephrase that: WE'LL ALL DIE!!! So the Pentagon recruits handsome Josh Keyes (Aaron Eckhart), professor of physics at the University of Chicago, who apparently got his graduate degrees at the George W. Bush School of Applied Language, because he pronounces his area of expertise noo-kew-ler.
And they recruit smarmy Dr. Conrad Zimsky (the formerly bald Stanley Tucci, whom the critic Elvis Mitchell describes as wearing Millard Fillmore's hair here), whose reputation was gained at the expense of his former collaborator Dr. Edward Brazzelton (Delroy Lindo), who by a remarkable coincidence has built a, well, a thing, that looks like a segmented worm fifteen feet in diameter with enormous lasers on the front, that - if things go well - will penetrate the earth's crust, its mantle and its core, carrying our intrepid crew of Keyes, Zimsky, Brazzelton, the Endeavor's copilot Beck Childs (short for Rebecca, and played by Hilary Swank), Beck's former commander Col. Richard Iverson (Bruce Greenwood), and the French scientist Serge Leveque (Tcheky Karyo, who seems to be the only one having a good time here, until you-know-what happens to him).
What will they do? They will drive this thing, which Dr. Brazzelton has made of a new metal called, yes, unobtainium, they will carry five noo-kew-ler bombs to be dropped within the core, they will restart the earth and save the world. Or they'll die trying.
Okay, it's a plan. After all, it's only three thousand miles to where they're going, with a temperature in the many many thousands of degrees, but fortunately the thing has windows in the front, so that even in the pitch darkness of the earth's core they can see where they're going. And did I mention that they can go outside the machine to repair it?
There's actually more, but you look like you're not all that interested, so I'll stop here. We do find out, near the end, that this was not a natural occurrence, but was caused by, yes, the Pentagon with a weapon that went awry. Four of the cast die heroically, two are saved, and if you're half as smart as I think you are, you'll guess which two. And as is now de rigueur in these films, the two are saved by young computer hacker Rat (DJ Qualls), who follows the songs of the whales. The end.