When 17 people suddenly drop dead within a 10-block radius in Boston, geophysicist Dr. Josh Keyes (Aaron Eckhart) and French atomic weapons expert Dr. Serge Laveque (Tcheky Karyo) are summoned by General Thomas Purcell (Richard Jenkins) to Washington D.C, to find out if the deaths are due to a covert enemy electromagnetic weapon. When it's determined that this was not an act of war, the government breathes a sigh of relief and the scientists are dismissed.
But Keyes can't let go of the deaths so easily, and when other bizarre "natural" phenomena begin occuring worldwide, he suspects the earth's electromagnetic field has been compromised. Working with his team at the University of Illinois, Keyes discovers the mystery behind the strange occurrences is more frightening than any act of war. For reasons unknown, the earth's inner core has stopped rotating, causing the planet's electromagnetic field to rapidly deteriorate. At the same time, life as we know it begins deteriorating as well, causing birds to lose their ability to navigate, whales to erratically change their migration patterns and people with peacemakers to suddenly drop dead. Even the famed Northern Lights appear in the night sky a hundred times brighter and farther south than ever before.
And things will only get worse. As the electromagnetic field, which shields the earth from solar radiation, slowly collapses, airplanes will start falling from the sky, and everything electronic will be fried. Static discharge in the atmosphere will create "super-storms" with hundreds of lighning strikes per square mile, and deadliest of all, microwave radiation will literally cook the planet.
Terrified by his findings, Keyes seeks out the opinion of renowned geophysicist Dr. Conrad Zimsky (Stanley Tucci), an arrogant scientist, who at first is unbelieving, but soon arrives at the same horrifying conclusion. Soon the two men, along with atomic weapons expert Leveque, are whisked away by federal agents to the Utah desert to meet Dr. Ed "Braz" Brazzelton (Delroy Lindo), a brilliant scientist who has been working for the last twenty years on a craft that can travel deep into the earth some day. But with no more time to reasearch, and with money suddenly no object, the U.S. government and the United Nations rally together to create a team of world experts that will build a fully functional subterranean craft -- immediately. And, because Keyes and his colleagues are scientists, not pilots, astronauts Major Rebecca "Beck" Childs (Hillary Swank) and Colonel Robert Iverson (Bruce Greenwood) are recruited to become the world's first "terranauts".
With their hastily built ship and their highly skilled, wildly diverse team of professionals to man it, the government and the world make the ultimate leap of faith...not into outer space but into inner space, journeying into unknown regions of the earth in the hopes of detonating a nuclear device which they hope will reactivate the core and restore balance to the planet.
Watching over the ship and crew on the earth's surface is Mission Control, led by NASA Control Chief Talma "Stick" Stickley (Alfre Woodard). With her is a room full of scientist experts and government officials, and unfortunately, an eccentric young genius called Rat (DJ Qualls). Formerly convicted of computer fraud, Rat is there to hack into the Internet to suppress information about the situation in order to prevent widespread panic. Secretly, however, he is also feeding Keyes covert data about something geophysicist Zimsky is hiding from the crew, something that the U.S. government is privy to and that could determine the entire teams's fate.
And so, as terranauts Iverson and Beck maneuver the subterranean ship -- appropriately named "Virgil" after a character in Date's Inferno -- through solid rock, and as they carve their way through a wondrous landscape of magma waterfalls, a mammoth geode full of crystals and a vast diamond field, the crew hardly knows what to expect.