of the large tank. It consisted of six partial and complete modules that took over half a year to plan and build from scratch. Can-Dive Services Ltd., a Canadian commercial diving company that specialized in saturation diving systems and underwater technology, specially manufactured the two working craft (Flatbed and Cab One) for the film. Two million dollars was spent on set construction. Filming was also done at the largest underground lake in the world—a mine in Bonne Terre, Missouri, which was the background for several underwater shots. Principal photography The main tank was not ready in time for the first day of principal photography. Cameron delayed filming for a week and pushed the smaller tank's schedule forward, demanding that it be ready weeks ahead of schedule. Filming eventually began on August 15, 1988, but there were still problems. On the first day of shooting in the main water tank, it sprang a leak and of water a minute rushed out. The studio brought in dam-repair experts to seal it. In addition, enormous pipes with elbow fittings had been improperly installed. There was so much water pressure in them that the elbows blew off. Cameron's cinematographer, Mikael Salomon, used three cameras in watertight housings that were specially designed. Another special housing was designed for scenes that went from above-water dialogue to below-water dialogue. The filmmakers had to figure out how to keep the water clear enough to shoot and dark enough to look realistic at 2,000 feet (700 m), which was achieved by floating a thick layer of plastic beads in the water and covering the top of the tank with an enormous tarpaulin. Cameron wanted to see the actors' faces and hear their dialogue, and thus hired Western Space and Marine to engineer helmets which would remain optically clear underwater and installed state-of-the-art aircraft quality microphones into each helmet. Safety conditions were also a major factor with the installation of a decompression chamber on site, along with a diving bell and a safety diver for each actor. The breathing fluid used in the film actually exists but has only been thoroughly investigated in animals. Over the previous 20 years it had been tested on several animals, who survived. The rat shown in the film was actually breathing fluid and survived unharmed. Production consulted with Dr. Kylstra on the proper use of the breathing fluid for the film. Ed Harris did