New York City. Forty men from Local 147 descend into City Tunnel No. 3 to continue construction on the city's massive underground aqueduct. They charge the dynamite. Take cover. A rumble. But this isn't the sound of dynamite. Before they have time to react, an underground earthquake rocks their world, wreaking havoc on the tunnel workers. Walls cave in and steam pipes burst loose. Some escape, others don't make it out alive.

Matt, the last man to flee the tunnel, sees something he can't believe - lava coming through the wall. The investigators, including Matt's ex-wife Susan, call it an impossibility. No one can figure out what caused the earthquake.

Meanwhile, in a top-secret warehouse across town, scientist Levering is performing an experiment to tap the Earth's core for energy. His crew is drilling to the centre of the earth around the clock with reckless abandon, affecting the delicate balance of the core in ways he could never have imagined.

Now the race is on - the Sandhogs from Local 147 need to stop the madman in time to save the city from certain destruction when the huge volcano that has been awakened unleashes its fury onto the streets of New York City. They come up with a brilliant plan to divert the lava into an existing aqueduct tunnel, and dig furiously against the clock to save the unsuspecting citizens of New York, emerging as silent heroes.