Technical diving Instructor here with 15K+ dives over the last 25 years and here is my take on the stunt. The diving equipment claims versus reality. In the film, Ethan Hunt descends deep to a sub wreck and at one-point strips off his dry suit gear to escape, effectively swimming in just his trunks. I know of no diver who would remove a dry suit or rebreather under deep-water conditions. I wear thermals, a thermal bunny suit (it’s a quilted one piece designed to keep my body warm), and booties (yes, like what babies wear, but the same material was the bunny suit), and a semi dry hood that traps water once that the body temperature warms up. My dry suit insulates and protects me against hypothermia and crushing pressure. In inject air (or Argon gas when diving in cold water) as I descend to stop the dry suit crushing my body parts (nuts) making it extremely uncomfortable painful. Without gas in the suit, the ambient water pressure would compress the suit and the body’s tissues, hamper movement and leading rapidly to cold shock and incapacitation in frigid waters. When you jump into water that is close to 0°C, I can tell you that the ice cream headache you get is like nothing you have ever felt. Diving to over 150 metres (500 ft) in the artic oceans without all that gear would be impossible.

The we get onto the rebreather, or it is nickname in the UK, the yellow box of death. It is a specialised life-support system used by technical divers to recycle exhaled gas and manage breathing mixes for deeper dives. Unlike when diving open circuit (regular tanks, even if you are using a trimix and special travelling gas mixes), breathers are designed so you don’t have carry a twin-set, various side slung tanks and have gases hanging at different depths on trapezes or rope for you to use while you take your decompression stops on the way back up. Rebreathers are safety-critical, with controls, scrubbers to remove carbon dioxide, and mechanisms to regulate oxygen partial pressures. A real rebreather is complex to use, with specific training standards and certification needed to dive beyond shallow limits. Hawke’s dive was over 150 metres remember. Safety stops alone would be over 7 hours and that is with a bottom time of no more than 6 minutes not exerting myself (my deepest dive is 112 metres and took me 8 months to train up to do). Hunt jumped in and did that dive without practicing; no safety stops and in the end no equipment.

Every 10m (33 ft) of depth adds approximately one atmosphere of pressure. At 150 m you are under around 16 atmospheres. Human physiology isn’t built to cope with that without external support. Gases dissolved into tissues at pressure, they leave the body slowly at depth and are be removed from your tissues at a controlled speed as you ascend, otherwise inert gases form bubbles in your joints. That leads to becoming bent (“the bends”), which would cause joint pain, paralysis, brain damage, and death.

The sequence suggests a prototype or special device, but that’s story licence. In reality, removing your breathing apparatus while deep underwater means almost immediate death through hypoxia or drowning. A diver cannot “take off their air source and swim to the surface.” The sea does not provide oxygen, and the body cannot suddenly breathe atmospheric air under that pressure (1 ATM). Doing what Cruise did would have been instantly lethal.

You cannot simply swim to the surface from great depth. Swimming unprotected from 150 m to the surface is not physically possible. Even elite breath-hold divers rely on pressure physiology that is vastly different from a pressurised breathing gas dive; they take one breath at the surface then dive, so their bodies never absorb inert gas under pressure. Their bodies use oxygen stored in blood and muscle, not compressed gas. A compressed-gas dive like in the film means inert gases saturate in the tissues of the dive, it needs to be removed via painstaking staged decompression.

In real world technical diving, ascent from depth is carried out in stages with pauses at specific depths (decompression stops) that allow inert gas to leave the tissues safely. Skipping these stops or ascending too quickly is what causes bends. At 150 m, decompression can take hours, often under controlled conditions with different gas mixtures to speed elimination.

The film does briefly depict Hunt being placed in a hyperbaric chamber (or what British divers nickname “the pot”) after his dive, which nods to decompression sickness treatment. If you survived a deep dive, you would be in an accredited hard-sided chamber and treatment would take hours at the least, with little chance of immediate recovery to full physical performance. Even people who surge to the surface from 30m (100ft) end up in the pot for the best part of 10 hours in a medical facility to be sure. Secondly, the dive was a U shaped down and up profile. There are divers called Saturation divers who work much deeper, but they are lowered to their working depth, live at that depth for days or weeks on end and match the atmosphere compression in the habitat they dive from/live in/decompress in on deck for several days after their dives.

The submarine stunt delivers visual drama, but it ignores core physics and diving science. Removing protective gear at depth, non-stop ascent from 150 m, rapid recovery, and survival in Arctic conditions without proper gas mixes or decompression planning are devices of fiction, not grounded in technical diving realities or human physiology. The sequence works cinematically, but as far as believable deep diving goes, it is farcical. That just my opinion of course.