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Sanctum
Seventeen-year-old Joshua "Josh" McGuire, expedition bank-roller Carl and his girlfriend, Victoria "Vic", travel to the Esa'ala Cave, an underwater cave exploration site in Papua New Guinea. Josh's father, Frank, a master diver, has already established a forward base camp at a lower level inside the cave, where the team has been exploring for weeks. The team below prepares to dive into an unexplored area of the system.
While exploring the entrance to the new system, Judes loses use of her air mask, forcing Frank to buddy breathe. Judes panics and tries to keep the mask on, but Frank forces the mask off of her knowing he will not have enough air to make it back to the team. As Judes drowns, Josh watches on a monitor at "forward camp" and presumes the worst of his father. Frank reveals that Judes had dived in an exhausted state since they had to retrieve the extra bailout tanks, a task Josh did not do. Meanwhile, their above-ground crew realises that a big storm is preparing to hit their location. They attempt to warn the team below, but are unsuccessful.
Wanting to return to the surface, Josh climbs back with Luko, Liz, and J.D, but water begins rushing in from their exit. The storm has turned into a cyclone, causing flash floods that begin to fill the cave. As J.D. and Liz make their way up through "the elevator" (an area leading up to the main entrance of the cave), Josh is unable to leave his father and the dive team behind to their doom and turns back with Luko. They discover that Frank and the team have already evacuated their camp and are assisting Victoria as she climbs up out of the cavern. Josh leaps in to help, strapping a rope around a nearby boulder and forming a belay. Unfortunately, the boulder begins to give way. The water rushes through and forces Victoria and Josh to fall back down into the flooded base camp. Luko is injured when the boulder breaks loose, sealing the shaft and throwing him back down into the cavern. He is swept into an underwater tunnel.
The team decides to use the unexplored tunnel as an escape route from the flooding cavern. Before the team can leave, a severely injured and mutilated Luko surfaces. Seeing that his friend is in pain and near death, Frank mercifully drowns him.
The team makes it through to the other side of the system. Having refused to wear Judes' old wetsuit, Victoria suffers from the cold water. Meanwhile, George—an experienced, veteran diver—has become ill due to the dive and is dying, unbeknownst to the rest of the team. The team continues through the system, following the flow of water out toward the sea. George realises that he cannot continue and hides himself so as not to burden the rest of the team.
The team arrives at a seemingly dead end. A hole in the bottom of the cave separates them from the other side of their path. Josh fishes a line across and begins to transfer their gear and each other. As Victoria begins to make her way across she catches her hair in her rope gear; she loses her grip, leaving her hair the only thing holding her weight. Using her knife, she attempts to cut away the hair, but severs her rope and falls to her death. At the sight of Victoria's death, Carl becomes emotionally unstable. In a fit of panic, he steals the last remaining rebreather and disappears into the tunnel. Josh and Frank find another way out through a crevasse in the cavern. The tunnel leads them to a sunlit cavern where an unidentified WWII Japanese tank collapsed through the surface years ago. Unfortunately, the hole in the roof where the tank fell is the only opening and they are unable to climb out. They spend the rest of the day there and proceed back into the cave by night.
They discover Carl, whose state of mind has worsened, having found Victoria's corpse. Carl attempts to murder Frank for Victoria's death but Josh separates the two. Frank has been gravely injured, having fallen on a stalagmite that punctured his back. When Carl wakes, he realizes the gravity of his actions and solemnly disappears into the tunnel. In pain, Frank requests that Josh drown him. Josh reluctantly does so, and swims into the tunnel.
He encounters Carl, who has already drowned looking for the exit. Just as he begins to lose hope, Josh discovers a way out through the cave to open ocean. He emerges from the water and crawls onto the beach, where he is discovered by fishermen.