After an earthquake, several Cavers map out a newly formed tunnel to an underground lake. Separated, one explorer falls victim to something in the dark water, while the others think she perished by falling through a crevasse to her death. A year later, the same lake is being explored by a team of seismologists and acoustical engineers to map the underground maze of lakes. Led by Doctor Guy Petrov and his right-hand woman, Anika, Petrov's small team is working with a submersible drone, fitted with just about every sound device known. The drone starts its dive, accompanied by Guy's fiancée, Faye.

Returning from the descent, Faye arrives topside just as the entire cavern comes under an odd sonic attack. She dies in the water before she can be saved, along with two park rangers. All drowned. Several seismologists are injured as part of the cavern collapses. The survivors split up, with a few climbing through a maze of deadly "rock crushing" obstacles to find help above ground. Guy, Anika and a college intern, Dinah, make their way topside to call for help and check on the recorded data to see what really happened within the water. Everyone above ground is missing, and they cannot place a phone call anywhere. When they analyze a sound byte of the unheard infrasonic waves which can kill, they discover it is a creature, or thousands of creatures, emitting the deadly sound from within the lake. Meanwhile, the others, led by Zimmerman, find an alternate way out: the Cavers tunnel.

Ready to save their friends, the three gather first aid supplies from a ranger station. But they know they need something to fight off the enemies from the lake. Inside the mining shed, used to excavate down to the mysterious lake, they are silently attacked by two shape mimicking creatures, both emitting the same unheard infrasound. As Dinah and Anika struggle with snake-like arms strangling them, it turns out the creatures are an unknown species of octopus that can live in and out of the water and use infrasound to stun or kill their prey. Barely surviving, the three find several flame throwers. They then head out to meet the others.

Zimmerman's group gathers in a few tourists, and all are followed and come under attack by the aggressive octopi. They try to survive but are overwhelmed. The battle turns once Guy arrives with the flamethrowers. Gaining the upper hand, the survivors journey back to the communications compound to put out another call for help. There is still no way to put through a distress call. It's then and there, they realize the octopi have left the underground lake and are hunting them. They decide to go back to the lake and seal it up.
Within the mining outpost they find dynamite they can use to collapse the cave. But they must split up again and return to the lake from both entrances, burning any octopi along the way. Once at the lake, they set up the blasting charges. Then another wave of octopi come forth from the water.

All but four lose the battle and are killed, either by accident or octopi. The cave is violently sealed after the four escape in time. All is then calm in the aftermath. Or, so the survivor's hope.