When a group of joy-riding underwater ETs decide to play chicken with a U.S. nuclear sub, its commander loses control and it plunges to the murky depths. It crashes into the bottom 2000 feet below the surface, floods, and drowns all aboard. Although the U.S. government is ignorant of the cause, it nevertheless finds the situation upsetting. It's the height of the cold war and the sub has gone down near Cuba. Those sneaky communists might get to the sub ahead of the Americans and steal their secrets. Obviously, quick action is needed.

The U.S. government has deep-water salvage equipment but it's not readily available, so it requests help from an experimental oil drilling rig which just happens to be located near the sunken sub. Unlike normal drilling platforms, this rig is designed to function on the ocean's floor and is far more mobile than a typical rig. It's equipped with a full contingent of deep-water divers and the equipment needed for a salvage operation.

A group of navy SEALs are dispatched to the rig along with the rig's designer. The seals have been given secret orders to arm a nuclear bomb and blow up the sub if there's any indication that it might fall into enemy hands. Evidently, government thinking is too primitive to foresee the possible large-sized wave, radioactive contamination, and PR nightmare resulting from such an explosion.

The SEALs have to pass through a pressure chamber before entering the drilling rig and are warned about the dangers of pressure-induced psychosis. Supposedly, it's impossible to predict who'll get it but it does have a number of noticeable symptoms such as hand tremors. After being pressurized, the SEAL's commander immediately takes on a bug-eyed look and exhibits hand tremors, but this apparently goes unnoticed by the crew until things have gotten out of hand.

As depicted in the movie, the drilling rig's atmosphere is kept in equilibrium with the pressure of the surrounding water. Indeed, this is about the only practical design for a large-scaled underwater drilling rig. Designing a rig which maintained one atmosphere of internal pressure at a depth of 2000 feet would be an expensive engineering quagmire. However, an equal pressure design also has its problems.

First, nitrogen in air starts acting like an anesthesia at a depth of about 150 feet (45.5 m). This is called nitrogen narcosis. Like tolerance for alcohol, tolerance for nitrogen narcosis varies from person to person but in the end its effects are the same. The person gets drunk as a skunk and eventually passes out. To solve this problem nitrogen is usually replaced with helium.

Second, at a partial pressure of around 2 atmospheres, oxygen becomes toxic to humans. Air is a mixture of gasses. The partial pressure of one of its components is found by multiplying the component's mass fraction times the air's pressure. Oxygen has a mass fraction of 21%. At 1 atmosphere of air pressure, the partial pressure of oxygen is 0.21 atmospheres. In 2000 feet of water the oxygen in air would have a partial pressure of over 12 atmospheres. To solve this problem the composition of breathing gasses would have to be about 1.7% oxygen for a partial pressure of 0.21 atmospheres.

All of this poses a considerable problem for moviemakers who want to portray realistic physics. The speed of sound is about 2.9 times faster in helium than air. Although the human voice is very complex, the resonant frequencies of air chambers and passageways in the throat and head play a significant role in how a voice sounds. Breathing a mixture containing 98.3% helium would increase these frequencies, making voices sound abnormally high-pitched. We must yield on this one to the moviemakers. It would be hard to take a movie seriously if the actors all sounded like chipmunks.

The SEALs borrow the drill rig's main submersible and go off to recover a nuclear warhead without bothering to tell anyone. Meanwhile, a hurricane rolls in and threatens the rig's support ship on the surface. The ship would simply sail away to safe harbor except that it's inconveniently tethered to the drilling rig by a large-sized cable. Naturally, the drill rig's main submersible is required for detaching the cable, and it's not available. The cable snaps off on the ship, pulling a large-sized crane with it in the process. Although the crane narrowly misses falling on the drilling rig, it nevertheless wreaks havoc.

Water gushes into the rig's compartments. This is pretty interesting since the gasses in the rig are at the same pressure as the surrounding water pressure. The hull would have essentially no stress in it from outside water pressure and would not be easily breached. For water to leak in, the same volume of gas would have to leak out. Small leaks would allow bubbles to escape externally and would cause water in the diver's portal to slowly rise. In a well-designed rig, an automated system would prevent the water level from rising too high by automatically opening valves on gas storage tanks ( holding helium and oxygen) to replace the lost gasses.

Larger leaks would allow water to dribble in through the leak itself. For really large leaks water would pour though the opening but at a far less dramatic rate than a similar leak in a typical military submarine. Military submarines generally have only 1 atmosphere of pressure inside. In deep water there would be a large pressure difference between the interior and exterior. Pressure induced hull stresses would also be very large. Water would gush into the sub even with a small leak. Breaches in the hull would would act as stress risers further increasing already high hull stresses and possibly leading to catastrophic failure. Air pockets in closed compartments would compress to much smaller sizes.

Absolute water pressure would be over 60 atmospheres at a depth of 2000 feet. Using Boyles law, we estimate that the volume of air inside a 1-atmosphere submarine would be compressed to 1.7% of its original size if water was allowed to enter the sub until the internal and external pressures were equal. This assumes that no air leaks out of the sub.

The moviemakers had evidently watched too many old submarine movies. The leakage problems in the drilling rig would have been very threatening but the biggest immediate issue would have been the loss of gasses rather than a rapid inflow of water. Flooded compartments would also very likely have contained fairly large gas pockets which could have kept trapped victims alive for long periods of time.

After getting the over-dramatized flooding under control, the rig's crew discover that not only are they completely isolated from the rest of the world but the whacked-out SEAL commander has brought a nuclear warhead onboard. He's also well-armed and in no mood for a discussion.

About this time, the aliens send a tentacle made of water into the rig to check things out. This is comic book  physics at its best. The tentacle can not only defy gravity and collect data about the humans, but also entertain them with cutesy tricks like mimicking their facial features.

This cutesy behavior fails to impress the SEAL commander and he decides to nuke 'em. This leads to fisticuffs and eventually a lengthy submersibles battle as the heroic drilling rig crew tries to stop him. During the battle, a robotic sub with the nuclear warhead attached breaks loose and heads over the side of an underwater cliff straight toward the aliens.

The battle leaves the SEAL commander dead and the heroes Bud and Lindsay Brigman ( Harris and Mastrantonio) trapped in a disabled and leaking submersible 60 meters away from the drilling rig.

Harris has the only scuba rig and it's not set up for buddy breathing. After some dramatic exchanges between the protagonists, the sub floods completely and Mastrantonio "drowns". (Don't get too sad. This is after all a movie.)

In reality the sub's internal pressure would have equaled external water pressure and water would have only filled to the leak's level. The leak was in a fitting behind a panel on the submersibles interior wall. When flooding reached this level, the breathable gas space in the submersible would still have been about 1/3 to 1/4 of the submersibles volume, giving Mastrantonio a chance for rescue without the overly dramatic drowning scene (assuming Harris didn't let the bubble out when he left to get additional breathing equipment).

The cold water temperature would have posed a significant threat to Mastrantonio as she waited for rescue. However, she was fully clothed and water is not a particularly good conductor of heat. On the other hand water rapidly removes heat if it's moving. By remaining relatively still, Mastrantonio's clothing would have trapped a stationary layer of water inside it, reducing her heat loss and extending the time before she died of hypothermia.

The ceiling in the submersible was relatively low forcing her to crouch when standing. This means she could have kept her head, shoulders and arms dry, again significantly reducing heat loss.

Mastrantonio would have had a good chance of surviving while Harris mounted a rescue. According to the United States Search and Rescue Task Force and the US Coast Guard, individuals in 0 to 4.4 degree Celsius water typically survive for 30 to 90 minutes (http://www.ussartf.org/cold_water_survival.htm#top, http://www.uscg.mil/hq/g-m/mse4/pfdcold.htm ). The water around the drilling rig would have been about 2 degrees Celsius and as mentioned above, Mastrantonio would have had a number of factors in her favor.

Back aboard the rig it's now clear that something must be done about the bomb which has gone over an underwater cliff and is sitting on the bottom about 20,000 feet (6100 meters) below the drilling rig. Unfortunately, this is far below scuba depth and there are no submersibles available that are capable of recovering the bomb.

Fortunately, the SEALs brought an experimental liquid-breathing deep-diving suit. Previously we had been shown scenes of a pet rat breathing while immersed in the special oxygenated perflouorocarbon (PFC) liquid. The rat scene was real and the rat unharmed. Indeed, the breathing liquid was actually under development at the time. However, it had a serious flaw for deep sea diving. The liquid was about twice as dense as water, making it exhausting to move in and out of the lungs when performing strenuous underwater work.

The Navy may now be using a secret deep diving system with liquid breathing, but according to world class submersible designer Graham Hawkes, such a system has never been commercialized. Even though the diving equipment was speculative, much to their credit, the Abyss makers hired knowledgeable consultants to make their portrayal of the technology reasonable.

Harris dons the equipment, drops over the ledge, and freefalls toward the nuclear bomb on the bottom. He holds a small robotic submersible in his hand which looks oddly like an oversized lunchbox. It's supposedly programmed to take him to the nuclear bomb attached to the other robotic submersible. As near as we can tell it doesn't do anything and eventually implodes.

The crew aboard the drilling rig called out the depth as Harris falls. By timing the fall we were able to construct a displacement verses time curve which turned out to be remarkably linear, indicating that Harris very rapidly reached terminal velocity and fell most of the distance at constant speed. When we ran it through linear regression analysis, it yielded a constant speed of 55.4 ft/s or 37.8 mph (60.5 kph) with an r-squared of 95.2%. In other words, the regression equation "explained" all but 4.8% of the variability in the data. It's remarkable that movie data would be so consistent. Unfortunately, a velocity this high would be roughly equivalent to falling off a four-story building (47.5 ft or 14.4 meters) with no air or water resistance. Yet, Harris touches down gently without injury.

On the bottom Harris immediately finds the nuke and sets about disarming it. Of course, it comes down to the old red-wire-green-wire gimmick, except this time it's the blue wire with a white stripe. Unfortunately, Harris only has a chemluminescent stick which gives off a yellow glow and seriously interferes with color perception. He has two wires to choose from, and, after a lengthy dramatic hesitation, gets the right one.

We're left wondering what kind of circuit would have two wires, one which disarms when cut and the other which detonates the bomb when cut. Only a goofball would design nuclear bomb circuitry with such nasty little surprises. Disarming a nuclear warhead should be made as foolproof as possible. The people who owned it definitely wouldn't want it to blow up if they were disarming it. On the other hand, an enemy agent wouldn't be able to disarm it after the warhead was launched.

Perhaps the timer attached by the Seals contained a booby-trap to make sure that spies didn't disarm the warhead. This still makes little sense. First, how would the spies find the bomb and reach it in enough time to deactivate it? Second, why set such a marginally effective booby-trap? There are three possible wire cutting options: cut one wire, cut the other, or cut both. Spies would probably rule out cutting both wires simultaneously since it could create a momentary short and trigger the bomb. This means the booby-trap would work at best only 50% of the time. Surely it's possible to come up with a more effective design.

After disarming the bomb, Harris reports that he has only 5 minutes of oxygen left. According to the drilling rig crew it took Harris 30 minute to reach the bottom. We had timed it at a little over 8 minutes. In either case Harris is obviously in big trouble. If he rises he must somehow slow down and stop by the time he reaches the ledge he went over. He then has to make his way at least an additional 60 meters to reach the safety of the rig. Even if his time of ascent matches our measurement of his previous time of descent, he's going to be at least 5 minutes short on his oxygen and likely black out before he reaches safety.

Harris decides to stay on bottom which proves to be a wise move. He's taken in by the aliens who place him in a room and do a Moses-style parting of the waters to provide him with a breathing space. They turn on the TV to make him feel at home and the reception is exceptional. This is all the more impressive since they're on the bottom of the ocean and no human has ever figured out how to transmit electromagnetic waves at TV frequency through thousands of feet of salt water. Apparently they have cable TV.

The aliens are pissed off about someone trying to nuke'em and decide on a little response of their own. They create 1000-foot-high (303-meter-high) tidal waves using some type of seismic activity and send them toward every shoreline on every continent.

Normally, water recedes ahead of a wave in order to provide the volume of water in the wave. In The Abyss this doesn't happen. Ships near the Golden Gate Bridge remain at their normal level as the tidal wave looms above them. This in itself is pretty interesting since tidal waves usually do not reach their great heights until they travel into the shallow water near shore.

We thought the tidal wave was totally impossible from the standpoint of energy alone, until we made some calculations. First, we assumed that the waves' shape could be modeled as a triangle whose height was 1000 ft (303 meters) and base was 500 ft (152 m). We assumed that there were a total of two pairs of waves, one in the Pacific and one in the Atlantic. For each pair, one would travel East and the other West. Each wave was assumed to have a length equal to 1/2 the world's circumference and a velocity of 100 m/s (225 mph). We calculated that the combined kinetic and potential energy of the waves was equal to the energy of about 1756 megatons of TNT.

We estimated that about 3512 megatons worth of nuclear bombs would have to be detonated to produce the waves. This assumed that half of the energy in each bomb was wasted as heat and that the bombs were properly placed. Yes, this is a lot of explosive power but is within the reach of a mere human superpower. Even if the energy requirements are underestimated, they would be, at least, conceivable for a highly advanced race which was capable of intra galactic travel.

Fortunately, the aliens are romantics and have eavesdropped on the loving exchanges between Harris and Mastrantonio when he lay at death's door. The aliens have a change of heart at the last minute and cause the waves to recede. Making the waves start was one thing, but stopping them and causing them to subside was downright silly. It means they would have had to remove the kinetic and potential energy from 1.23×10¹⁵ kg of water. This is like trying to put a stick of dynamite back in the box after it's exploded.

Virtually every aspect of the aliens is based on fairy-tale logic and comic-book physics. Their underwater runabouts travel at incredible speeds creating highly turbulent wakes. These were not only easily detected by the underwater listening device in the nuclear submarine that sank, but tossed the sub around like a Dixie cup.

When the aliens passed by, lights dimmed and electrical equipment inside the sub went haywire although the continuous metal shell of a sub immersed in sea water would have afforded excellent shielding from stray electric fields. Military electronics would also have been specifically designed to resist the effects of unwanted electrical noise.

During the Cold War, sensitive listening devices along the coast of America could identify ships leaving harbor on the other side of the ocean by detecting their propeller noises. In addition, the US Navy had ships all over the world listening for submarine activity. Supposedly, none of these detected the alien runabouts.

When the drilling crew is finally brought to the surface they miraculously don't need to decompress. They have not died of asphyxiation from breathing a 1.7% oxygen mixture on the way up and fortunately, they remembered to exhale as they rose. Failure to do so would have caused their lungs to explode as the gasses in them expanded when the pressure was reduced.  The explanation for these miracles: why it must be the aliens. Evidently they have magical powers. Talk about a fairy tale ending.

It's a real shame that a fine effort like The Abyss degenerated into comic-book silliness. The movie-makers took almost loving care in their set design and attention to underwater details. In this regard The Abyss is the premier underwater movie. Yes, it had some definite physics slipups, but it also had compelling drama and tension. It didn't need magical aliens to make it entertaining.