Sanctum

Seventeen-year-old Joshua "Josh" McGuire (Rhys Wakefield), expedition bank-roller Carl Hurley (Ioan Gruffudd) and his girlfriend, Victoria "Vic" Elaine (Alice Parkinson), travel to the Esa'ala Cave, an underwater cave exploration site in Papua New Guinea. Josh's father, Frank (Richard Roxburgh), 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. As Josh voices his disdain for his father and his opinions about cave exploration, the team below prepares to dive into an unexplored area of the system.
While exploring the entrance to the new system, Judes (Allison Cratchley) experiences a problem with her air tank hose. She loses use of her air mask forcing Frank to buddy breathe. After a few exchanges, Judes panics and tries to keep the mask on, but Frank forces the mask off of her knowing he will not have enough air otherwise 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. In a struggle to determine who truly was responsible for her death, it is revealed by Frank that Judes had dived in an exhausted state since they previously had to retrieve the extra bailout tanks, a task Josh didn't do. Meanwhile, their above-ground crew realizes that a very big storm is preparing to hit their location sooner than anticipated. They attempt to warn the team below, but are unsuccessful. Josh expresses his desire to return to the surface and with communications down and an uncertain expedition in front of them, team leader Frank agrees to have his son return to the surface with a buddy climber ahead of them.
While Josh climbs back towards the surface with Luko, Liz, and J.D, water begins rushing in from their exit. The storm they were trying to avoid had 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, accompanied by Luko. As they are making their way back to "forward camp" they discover that Frank and the team have already evacuated their camp and are assisting Victoria as she climbs her way up and out of the cavern. Josh leaps in to help, strapping a rope around a nearby boulder and forming a belay for the team to escape. Unfortunately, the boulder begins to give way leaving Josh to hold Victoria's line and Luko to hold back the boulder. The water rushes through and forces Victoria and Josh to fall back down into the now flooded base camp. Luko is severely injured when the boulder breaks loose, sealing the shaft and throwing him back down into the cavern. He is swept into an underwater tunnel before the team can reach him.
The team decides to use the unexplored tunnel as an escape route from the rapidly flooding cavern. Victoria, panicked and anxious, refuses to wear Jude's old wetsuit and is given a quick tutorial on how to dive. Before the team can leave, a severely injured and mutilated Luko surfaces in the cavern. Seeing that his friend is in great pain and near death, Frank mercifully drowns him. The team presses on and makes it through to the other side of the system. Without a wetsuit, Victoria suffers severely 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. After a short rest, the team continues through the system following the flow of water out towards the sea. George realizes that he cannot continue and after sharing a few words with Josh relieves himself of his pack and hides himself so as not to burden the rest of the team.
As the team marches on they arrive at a seemingly dead end. A great hole in the bottom of the cave separates them from the other side of their path. Josh uses his climbing expertise to fish 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 is hung on and loses her grip, leaving her hair as 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 psychotic. In a fit of panic, he steals the last remaining rebreather and disappears into the tunnel. Josh and Frank, however, 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 middle of 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.
A little later they discover Carl who has gone completely insane and found Victoria's corpse. Carl attempts to murder Frank but Josh separates the two and temporarily disables Carl. Frank, however, has been gravely injured having fallen on a stalagmite which punctured his back. When Carl wakes, the gravity of his actions is realized and he solemnly disappears into the tunnel. In immense pain, Frank requests that Josh drown him. Josh reluctantly does so, and swims into the tunnel. A short distance in, 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.
The end credit paid a tribute to Wesley C. Skiles.

Question: Frank is gravely injured when he falls on what?
Answer: a stalagmite