The Tunnel Movie

In the midst of the drought and water shortages, the New South Wales State government has unveiled plans to recycle millions of litres of water trapped in a network of abandoned train tunnels beneath the heart of Sydney. However, the government suddenly goes cold on the plan and it does not tell the public why. There is talk of homeless people who use the tunnel as shelter going missing. This and the silence from the ministers leads a journalist Natasha, to begin an investigation into a government cover-up. She and her crew Peter (producer), Steven (Camera operator) and Tangles (Audio engineer) decide to investigate the story and plan to enter the tunnels themselves. After being refused entry to the tunnels by a security guard, they find an alternate entry and make their way inside. They proceed to explore the tunnels and locate various abandoned homeless squats and sections used as air raid shelters in the 1940s.
They come upon a huge underground lake and while filming, Tangles hears strange noises through the audio headphones. Later, the group heads to the room where the WWII air raid bell is. To get footage of the bell ringing, Natasha hits it, but Tangles says it is distorting the audio. He decides to take the boom-mike into an adjoining room to reduce the sound levels, leaving Peter with the headphones to listen for distortion in the bell volume. Natasha hits it again and Peter hears something very disturbing in the headphones. Tangles' audio cable is suddenly pulled through the opening, snapping and disappearing. The rest of the crew panics and goes to look for Tangles.
The scene switches to the interview, where Natasha is allowed to hear this recording for the first time. As the sound of the bell plays, screams of agony can be clearly heard before the headphone wire snaps. Upon going back to the bell room, the crew finds that all their equipment have disappeared. While searching for Tangles, they find a room blocked off by corrugated metal. Natasha places down the camera to help Peter and Steve. They get into the room and are horrified to find blood everywhere and Tangles' abandoned torch. At the entrance of the room, Natasha picks up the camera, seeing that it was not where she had left it. Reviewing the footage, it is revealed that something had picked up the camera and is stalking them.
The crew ventures through the tunnel, trying to find a way out. They hear noises of a person calling down, and come upon the security guard that had earlier refused them entry. They tell him that one of their crew has gone missing but the guard tells them to quickly follow him and get out. Something pounces on the guard as he nears a corner, dragging him out of sight. The group flees, screaming in terror through the narrow tunnels. They hide in a room and stay there until they decide the creature has gone. Trying to find a way out, they return to the underground lake. Hearing something, they extinguish all light and use the night-vision camera to look towards the sound. There, they discover the security guard, his eyes missing and whimpering in pain in the shallows of the lake. The crew watches in horror as a tall, emaciated humanoid appears behind him and murders him by brutally twisting his neck until it breaks. Natasha lets out a scream, attracting the creature's attention, and the terrified group runs.
They come to a dead-end with a barred opening into a street above them, and continue searching for an exit. At one point, the creature attempts to grab Peter. However, as Steven shines the camera's light on them, the creature immediately retreats. Steven realizes that the creature is vulnerable to light. Upon reaching another dead-end, Natasha discovers a side route. Entering this new room, they find signs of some living being, and a small pile of human flesh. The main camera loses power, causing Steve to frantically attempt to replace the battery. Just as the power switches the light on again, the creature attacks, knocking Peter and Steven to the ground. Natasha, panic-stricken, runs off with the night-vision camera. On her own, she stays in silence, but is confronted by the creature, who cracks the camera lens as it grabs at her, dragging her with the small camera still recording. Steven and Peter follow the sound of her screams and the creature's movements.
Through the night-vision camera, it is shown that Natasha is dragged back to the lake. The creature attempts to drown Natasha but before it can succeed, Steven and Peter use the light to chase it away. As Steven helps Natasha, Peter is heard screaming and challenging the creature, drawing it away from the other two. Steven manages to drag Natasha to a well-lit tunnel near a train station. They rush back and drag a badly-wounded Peter to the safety of the train platform. There, captured on CCTV, Natasha begs for help. The crew ends up staying in the tunnel all night.
In the epilogue, it is stated that the coroner found that Peter died from extensive internal bleeding. Natasha resigned after the underground shoot, no longer working as a journalist. Steven still works as a news cameraman. Tangles' whereabouts remained unknown and his family are still searching for answers. The police investigation was closed due to 'contradictory evidence'.

Question: What did the crew found while filming?
Answer: They come upon a huge underground lake and while filming.