Why did it have to be unobtainium? As if James Cameron's use of this lame science fiction cliche in AVATAR (2009) wasn't bad enough (and believe me, it was pretty damned egregious), the same wonder element was used six years earlier in THE CORE, another sf action adventure film with good special effects, cardboard characters and a formulaic plot.
Something has caused the earth's core, an immense, rotating sphere of electro-magnetic energy, to begin slowing down. As the cores rotation slows, it wreaks havoc with the earth's atmospheric electro-magnetic field allowing deadly solar radiation to penetrate the atmosphere. The intense microwave bombardments cook much of San Francisco and a hellacious lightning storm levels Rome.
The only way to combat this menace is to detonate a series of atomic bombs within the core with the hope that the explosions will re-start the rotation and return things to normal. The problem is, how do you deliver the payloads into the center of the earth? The answer is an experimental vessel, dubbed The Virgil, a train-like ship with six segmented "cars", each one containing a bomb. The ship is equipped with a laser-driven boring system and is built out of unobtainium, which can withstand the incredible heat and crushing pressure of the earth's crust and convert those forces into energy.
A six person crew is assembled to pilot The Virgil. The leader of the expedition is Dr. Josh Keyes (Aaron Eckhart). He's accompanied by fellow scientists Dr. Edward Brasselton (Delroy Lindo), Dr. Conrad Zimsky (Stanley Tucci), and Serge Leveque (Tcheky Karyo). Two NASA space shuttle pilots, Commander Robert Iverson (Bruce Greenwood) and Major Rebecca Childs (Hilary Swank) will drive the vehicle. On the surface, the team has a world class computer hacker, Rat Finch (DJ Qualls), who looks like a human Golum and mission control chief Alfre Woodard.
The Virgil and its' crew encounter various dangers and some nefarious plot twists and turns. By the time the ship has reached the core, only two crew members are left to save the world. Of course, they do.
THE CORE combines elements of '70s disaster films with decent special effects. The screenplay by Cooper Layne and John Rogers is by-the-numbers but director Jon Amiel does a decent job keeping things moving. It's not a bad film, but it got mixed reviews and performed very poorly at the box office.
If you've got an itch to scratch regarding films which take place beneath the surface of the earth, I'd suggest JOURNEY TO THE CENTER OF THE EARTH (1959), CRACK IN THE WORLD (1965), BATTLE BENEATH THE EARTH (1967), BENEATH THE PLANET OF THE APES (1970) and AT THE EARTH'S CORE (1976). Or, if you really want some first rate adventures inside the earth, track down and read some of the old issues of DC's SHOWCASE comic book series featuring this guy: