Bayona described digitally combining the on-location footage with the footage filmed in the tank as "kind of a puzzle. ... We were working for months on every shot. Every shot could have tons of layers."
Capturing the right sound for the 10-minute tsunami sequence was also critical. Bayona wanted supervising sound editor Oriol Tarragó to create a sound design that wouldn't have any music. After many experiments, Tarragó came up with the concept of having a different sound for each shot of the tsunami sequence.
"Every shot has a different point of view, so I tried the idea of making a contrast between every shot, like high and low frequencies and volumes," he said. "We collected different sounds from waterfalls and underwater recordings."
Conversations with Maria Belon, the Spanish woman played by Watts, helped inform Tarragó's choices. "Maria was telling us she didn't know what was going on [as the tsunami approached]," said Tarragó. "She thought that a plane was flying really low. So we recorded a lot of planes here in Barcelona."
Tarragó also used "sounds like a far-off plane flying when the water is coming to the resort. We also used the vibration sounds of glass and animals running and birds flying because they could feel what was going on."
Belon, says Bayona, was "very impressed" with the sequence when the water comes in.
"But she was really shocked with the sequence where you see her underwater, because for her that was the perfect metaphor of what was life, how you cannot control your destiny," he said. "You are only dragged by the current."
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