Fluid breathing is a reality. Five rats were used for five different takes, all of whom survived and were given antibiotic shots by a vet. The rat that actually appeared in the film died of natural causes a few weeks before the film opened. According to James Cameron, the scene with the rat had to be edited out of the UK movie version because "the Royal Veterinarian felt that it was painful for the rat". James Cameron repeatedly assures that the rats used for this take didn't suffer any harm.
Director James Cameron contacted Orson Scott Card before filming began with the possibility of producing a book based on the film. Card initially told his agent that he doesn't do "novelizations", but when she told him that the director was James Cameron, he agreed to consider it. The script arrived, and Card signed on after receiving assurances from Cameron that he would be free to develop his "novel" the way he wanted to. After a meeting with Cameron, Card immediately wrote the first three chapters, which dealt with events concerning Bud and Lindsay Brigman that occurred before the events in the film. Cameron gave these chapters to Ed Harris and Mary Elizabeth Mastrantonio, who used it to develop their characters.
The masks were specially designed to show the actors' faces, and had microphones fitted so that dialogue spoken at the time by the actors could be used in the film. The noises made by the regulators in the helmets were erased during sound post-production.
The crew frequently spent enough time underwater to force them to undergo decompression before surfacing. James Cameron would often watch dailies through a glass window, while decompressing and hanging upside down to relieve the stress on his shoulders from the weight of the helmet.
The tank was filled to a depth of 40 feet, but there was still too much light from the surface, so a giant tarpaulin and billions of tiny black plastic beads were floated on the surface to block the light. During a violent storm the tarpaulin was destroyed, thus shifting production to night time.
James Cameron's brother, Mike Cameron, plays a dead crewman inside the sunken submarine. To accomplish this he had to hold his breath under 15 feet of water while also allowing a crab to crawl out of his mouth.
Very few scenes involved stunt people. When Bud drags Lindsey back to the rig, that's really Mary Elizabeth Mastrantonio holding her breath. When the rig is being flooded and characters are running from water, drowning behind closed doors, and dodging exploding parts of the rig, those are all actors, not stunt people.
The scene with the water tentacle coming up through the moon pool was written so that it could be removed without interfering with the story, because no one knew how the effect would come out. The actors were interacting with a length of heater hose being held up by the crewmen. When the effects were completed, though, they exceeded everyone's expectations and wildest hopes.
During the rigorous and problematic shoot, the cast and crew began calling the film by various derogatory names such as "Son Of Abyss", "The Abuse" and "Life's Abyss And Then You Dive". Mary Elizabeth Mastrantonio reportedly suffered a physical and emotional breakdown because she was pushed so hard on the set, and Ed Harris had to pull over his car at one time while driving home, because he burst into spontaneous crying.
The original theatrical version was forced to cut the pre-credits quote "...when you look long into an abyss, the abyss also looks into you" by Friedrich Nietzsche because Criminal Law used it, and they didn't want to seem like imitators. The quote was restored in the director's cut.
For financial reasons, the "Deepcore" set was never dismantled. It stood in the abandoned (and drained) South Carolina nuclear power plant, where the film was shot. 20th Century Fox had posted signs around the set informing potential photographers that Fox still owned the set (and the designs) and that any photographs or video shooting of the set was prohibited by copyright law. Their official copyright information was on the Deepcore rig itself. A favorite destination for "urban explorers", the sets and facility were eventually demolished in 2007 during a reconstruction project.
The American Humane Association rated this film "unacceptable" because of the rat that was submerged in oxygenated liquid in one scene. It wasn't an effect. The rat really was "subjected to the anxiety of being submerged in this liquid, where it panics and struggles and is then pulled out by its tail as it expels the liquid from its lungs."
The sequence in which Catfish fires a submachine gun into the moon pool at a departing Lt. Coffey was filmed using live ammunition. The underwater camera was locked down and unmanned, and extreme safety precautions were in effect.
A scene at the beginning showing the crew rounding up at the moon pool had to be re-shot, because the Flatbed submersible was parked in the pool. Flatbed was supposed to be out in the water pulling the rig during that particular scene.
In the original storyline, when Lindsey is talking to Bud during his descent, she explains why she is always so hard on people. Lindsey grew up in a family with five older brothers, and she had to fight for everything, even to be noticed.
The scene with the water tentacle was one of the first to be filmed. This was done so as to give the effects team the maximum amount of time available to develop the CGI over the course of filming the rest of the movie.
One of the first films to make proper use of CGI technology. The animated water effects would be put to use in James Cameron's next film Terminator 2: Judgment Day, to create the liquid Terminator, the T-1000.
The extended scenes with the aliens were cut from the theatrical release because it would have made the film almost three hours long. And back in 1989, a running time that long was considered a big commercial risk. Especially for a film with lots of action scenes and special effects. It increases the likelihood the film won't turn a profit. The scenes were eventually restored in 1993 for the Special Edition.
The idea for the film came to James Cameron when he attended a science lecture about deep sea diving in high school. He wrote a short story about a group of scientists in a laboratory at the bottom of the ocean. But a film about a group of scientists didn't seem commercial to him so he changed it to a group of blue collar workers instead. Ironically Ghostbusters, a film about scientists was the No 1 hit of its year, while The Abyss did only middling business.
During the resuscitation scene, Ed Harris wasn't acting to Mary Elizabeth Mastrantonio in many of the shots. He was yelling at thin air. During the scenes she did appear in, Mastrantonio stormed off the set because the experience was so traumatic. She learned afterwards the camera was out of film and had to do it over again.
During underwater filming, Ed Harris almost drowned. While filming a scene where he had to hold his own breath at the bottom of the submerged set, Harris ran out of air and gave the signal for oxygen. Harris' safety diver got hung up on a cable and could not get to him. Another crew member gave Harris a regulator, but it was upside down and caused him to suck in water. A camera man came over, ripped the upside down regulator, and gave him one in the correct orientation. Later that evening, Ed broke down and cried.
During the segment where the mini-subs are exploring the sunken nuclear sub the actors can be seen inside the model mini-subs. This was achieved by putting a tiny screen and projector inside the models, projecting movies of the actors.
Most reporters seen on television in the extended edition of the movie are or have become James Cameron regulars. The man called Bill Tyler seen reporting from one of the ships is William Wisher Jr., Cameron's long-time friend and co-screenwriter who made several cameo appearances in his movies. Anchorman Joe Farago also appeared as anchorman in The Terminator. The reporter seen in the finale (Tom Isbell) also appears in the finale of True Lies.
Captain Kidd Brewer Jr. was already a professional diver prior to filming, and had also appeared in Cameron's Piranha Part Two: The Spawning. He tragically died during a diving accident one year after the premiere of 'The Abyss'. The Special Edition is dedicated to his memory.
When Lt. Coffey retries the keys from the captain's corpse, the name Kretschmer is visible on the name tag on the front of his overalls. This is a reference to the real-world Otto Kretschmer, the highest-scoring submarine ace of World War II.
'Ed Harris' has publicly refused to speak about his experiences working on the film, saying "I'm not talking about The Abyss and I never will". Similarly, Mary Elizabeth Mastrantonio said "The Abyss was a lot of things. Fun to make was not one of them."
Real oxygenated fluorocarbon fluid was used in the rat fluid breathing scene. Dr. Johannes Kylstra and Dr. Peter Bennett of Duke University pioneered this technique and consulted on the film. The only reason for cutting to the actors' faces was to avoid showing the rats defecating from momentary panic as they began breathing the fluid.
The trivia items below may give away important plot points.
In the end shot where the alien ship surfaces, it's supposed to be spring or summer. However, the film was being shot towards the beginning of winter, so the actors put ice cubes in their mouths so they wouldn't breathe out mist.
The final shot of the movie is of Bud and Lindsey embracing each other, but they aren't played by Ed Harris and Mary Elizabeth Mastrantonio. The shot was filmed with two extras after principal photography was completed.