A volcano beneath New York City seems unlikely, yet nothing else can explain the bizarre tremors and terrifying explosions wreaking havoc on the city. Matt McLaughlin, a tunnel digger and head of the Sandhogs, has witnessed lava seeping into the city’s aqueduct system and understands there is a terrifying truth at work behind the chaos. A group of tramps trying to keep warm in Central Park near a heat vent dies from carbon dioxide and sulfur dioxide poisoning, and a harbor boat is blown apart by a vent explosion. The FBI Terror Task Force begins pursuing the supposed terrorists who killed the tramps and blew up the boat, even as the city reels from the unfolding disaster.
Matt is pulled off the Sandhogs after several of his men suffer serious injuries, triggering an internal investigation. Pipes that should carry water are instead filled with steaming acid, another sign that something catastrophic is happening beneath the streets. He reunites with his former wife, Susan Foxley, who has been assigned to interview him about the events, and she comes to believe there is something seriously wrong in tunnel number three. Together they venture into sealed-off sections of the subterranean network, where Susan sees enough to understand Matt’s warnings.
Andrew Levering has been advancing a geothermal energy project that has somehow triggered the volcanic activity. His operation digs seven miles into the earth from a top-secret location hidden in a downtown warehouse, and his financiers, Neil Kavanagh among them, push for quick results. With no successful outcomes, they threaten to pull funding, forcing Andrew to push further and take greater risks in hopes of delivering instant proof.
Back on the surface, Matt and Susan press the Mayor for action, but Susan’s boss, Jacob Reed, leaves them frustrated and the Mayor dismissive. Neil argues against the possibility of a volcanic threat, lending weight to the skepticism. The situation intensifies when a homeowner is suddenly buried alive inside his house by a torrent of molten lava, and a whole block disappears as flames swallow homes—72 people go missing, and fear grows that the worst is only beginning. Agent Walters of the FBI’s Terror Task Force is granted broad authority to act as the city trembles.
The couple follows a flicker of heat with a handheld thermal detector and traces an unnatural warmth to a warehouse. They enter, but Andrew recognizes Matt and captures them, turning the situation into a trap. In a bid for results, Andrew’s obsession spirals out of control; flames erupt from the drill hole, the workers desert the project, and the warehouse explodes. The following days reveal lava spurting through manholes and buildings, transforming streets into a landscape of fire and destruction that costs thousands of lives and untold damage.
From a window high above, the Mayor witnesses the devastation, and Neil reluctantly concedes his involvement. An evacuation of the city is finally ordered as the scale of the catastrophe becomes undeniable. Matt, quick-thinking and determined, devises a plan to redirect the lava by detonating charges in the right places to channel it through the aqueduct tunnels and out to the sea. The clock ticks as the crew races to implement the plan, all while Andrew—now disfigured and driven by a distorted belief in the project’s viability—stalks the tunnels with a gun.
In the claustrophobic tunnels, Matt and Susan confront the danger as they set the charges that could save New York. Andrew closes in, shoots Matt, but a misdirected shot destabilizes a wall and releases a surge of steam, briefly blinding him. The two manage to escape the collapsing tunnels just as the eruption reshapes the city’s fate. Andrew is killed in the final lava surge, and the plan succeeds: the lava is diverted, the city is saved, and New York begins the slow process of healing from a crisis that could have ended in utter catastrophe.
