PHANTOM OF THE OPERA star Emmy Rossum had to face up to her biggest fear on the set of new disaster film POSEIDON - swimming underwater. The actress took on the role in the sinking ship drama knowing she'd have to take scuba diving lessons and learn how to stay calm when holding her breath for long periods of time. She says, "It was probably the hardest thing I ever had to do. I had to learn to scuba dive and free dive. I had to overcome my personal fear of scuba diving because I'd only been once on vacation and had a total panic attack underneath the ocean. "I was hyperventilating and all my friends were saying, 'Look at the pretty fish.' I was like, 'Get me out of here!' "Day one they trained us in this contraption that they called the cage of death, which was this plexi-glass cube that they would sink over my head until I was gasping for air at the last minute. "It would fully submerge and I would be under there and have to hold my breath as long as I could, which, by the end, was a minute and a half... Then I would give a sign that I was really dying and they would lift the cube and I would swim up to the top. "To make matters worse, you can't hear 20 feet underwater but (director) Wolfgang Petersen's voice is blasting through underwater speakers."