One of the scariest thoughts is to imagine yourself inside a submarine that is sinking
uncontrollably. You likely know that given sufficient depth, the vessel will implode
due to the increasing pressure exerted by the surrounding water. That said,
I was shocked when I learned more about what actually happens when a submarine or any
submersible reaches its maximum operating depth, also known as its “crush depth”.
But how it's possible to estimate the depth of a submarine implosions based on the sound that
it makes, why it’s highly unlikely to recover the bodies of those onboard, how the crew of a sunken
submarine were once rescued from the bottom of the ocean, and why no one, and I mean no one,
has actually ever experienced the implosion of a submarine, is Not What You Think.
On November 15th, 2017, the Argentinian Submarine, ARA San Juan disappeared a few hundred miles off
the coast of Argentina. About a week later, a report was published by the Comprehensive Nuclear
Test-Ban Treaty Organisation. They claimed that they had detected a hydro-acoustic anomaly about
30 nautical miles north of the sub’s last-known location. The anomaly had happened a few hours
after the submarine’s last contact, and the assumption was the acoustic signal had been
produced by the collapse of the pressure-hull of ARA San Juan. But the report also stated the depth
at which the submarine hull had collapsed, and I was curious; how did they figure out the implosion
depth given that the collapse would have happened way before hitting the bottom of the ocean?
During an underwater implosion or explosion, the gas bubble that is inside the structure
oscillates, collapsing and expanding continuously, before it dissipates. This is known as the “Bubble
Pulse Effect”. The frequency of the pulse can be measured acoustically, and since the
volume of air inside the submarine is known, the depth of the collapse event can be calculated.
In the case of ARA San Juan, the bubble pulse frequency was about 4.4 Hz and the
implosion depth was calculated to be at 1,275 feet underwater.
The derived depth value can then be used to determine the energy required to produce the
acoustically-detected frequency at that depth. In this case, the energy released by the collapse
was equal to the explosion of 12,500 pounds of TNT. The surrounding water pressure was 570 PSI,
and the submarine hull would have collapsed at over 1,500 miles per hour.
That sounds terrifying, and yet, no one has actually experienced it.
Of course, many have tragically lost their lives in such accidents,
but none of them would have had the time to feel or comprehend it.
It could take anywhere from 100 milliseconds to 2 seconds for the human brain to experience
pain. That’s because there is a delay for the sensation to reach the brain, and also a delay
for the brain to perceive it. With dull pains, like when you stub your toe, it could take about
1 second until you actually feel the pain. But if you burn your fingers, your brain experiences
that a lot faster, in hundreds of milliseconds. No need to try it at home. Someone already has!
It was estimated that the pressure-hull of ARA San Juan was completely destroyed in about 40
milliseconds. That’s less than half the time for anyone on the submarine to consciously experience
anything, including pain. Even though the crew may have been aware that a collapse was imminent,
they never experienced it as it was occurring. Their deaths would have been instantaneous.
As for the bodies of those onboard, they cannot really be recovered.
The collapse of a submarine’s pressure hull has some similarities to a diesel engine,
in which the movement of the piston compresses the air and the diesel fuel in a short period of time.
The extreme pressure causes diesel fuel to autoignite in the engine.
Similarly, the air inside a sub could have fairly high concentrations of hydrocarbon vapors.
Things like hydraulic oil, diesel oil from the auxiliary diesel engine,
grease and rubber sublime to make their way into the submarine’s atmosphere.
When the hull collapses it behaves somewhat like a very large piston on a very large diesel engine.
The air can auto-ignite. And even if the air doesn’t ignite,
the extreme compression would make it extremely hot. The sheer force of implosion followed by the
oscillations of the bubble pulse effect will not leave any bodies behind to be recovered.
That said, there have been instances of people who’ve made it out of a sunken submarine alive.
For depths up to 600 feet, special submarine escape immersion suits
can protect the crew while they use an escape hatch or a torpedo tube to get out.
The ascent from 600 feet will only take 3 to 4 minutes, but it’s an extremely traumatic
experience, involving panic, oxygen narcosis, and perforated ear drums.
But things get much worse when the sub is too deep to use an escape suit. Your only chance
of survival would be a Submergence Rescue Vehicle like the Russian Priz-class vessel;
a titanium hulled vehicle that can rescue up to 16 people at a time from a depth of up to 3,200 feet.
Some submarines like the Russian Typhoon class are equipped with an
escape pod, but their reliability in actual emergencies have been questionable at best.
The rescue attempt of the Russian Kursk Nuclear submarine involved
several submergence rescue vehicles, but unfortunately, the mission failed,
due to the inability of Priz to dock onto the stranded submarine.
These complications are what makes the rescue of the crew of USS Squalus a near miracle.
In May of 1939, on her 19th test dive, USS Squalus submerged, but due to a malfunction,
the main air induction valve opened when Squalus was 60 feet underwater.
This caused the flooding of the aft torpedo room, both engine rooms,
and the crew's quarters, sinking the submarine to the bottom of the ocean.
Those who were in the sealed compartments had enough air to breathe for 48 hours at best.
Cut off from outside communication, the crew released a buoy from the deck,
which had a telephone attached to it, in the hopes that the rescue team would find it. And now, all
they could do … was keep calm and wait. No rescue attempt of sunken submarines had ever succeeded
beyond 40 feet. The crew of Squalus were sitting on the ocean floor, 243 ft below the surface.
Some time later, the buoy was spotted by their sister boat, Sculpin.
The two commanders were able to exchange a couple of words,
but an ocean swell caused the line to snap. No more communication was possible.
Within 24 hours, rescue ships had arrived and they had an experimental device to deploy. It
was a rescue bell. A hard-hat diver had to first get ready and descend, to carry a downhaul cable
from a winch inside the rescue bell. Once the cable was connected to the sub, the
bell was lowered into the water and then placed exactly above the hatch of the sunken submarine.
Stranded on the bottom of the ocean, the crew of USS Squalus were thrilled to greet the rescuers.
7 sailors climbed into the bell and were then brought up to the surface.
Three more trips had to be completed before all 33 men were rescued.
But the US Navy spent another 113 days salvaging the submarine itself. There were bodies in the
sub that still needed to be recovered. The plan was to attach pontoons to the hull
of the submarine in order to raise it off the ocean floor and then transport it back to port.
To do so, the pontoons had to be first filled with water to create negative buoyancy and
descend into the water. Once attached to the sub, air was pumped into the pontoons, which
pushed the water out, making the pontoons buoyant. During the first attempt, the pontoons attached to
the bow raised too quickly, causing the bow to rise out of water and slip out of the cables.
Eventually, USS Squalus was towed back to port on September 13th, 1939. Twenty-five
bodies were recovered from the wreckage. The body of the 26th victim was never found.
In less than a year, Squalus was repaired and recommissioned under the name USS Sailfish,
which served during WWII. The crew were forbidden from uttering the word “Squalus” while onboard
the Sailfish. After decommissioning in 1945, the conning tower was cut away and placed in a park
at the Portsmouth Naval Shipyard, where memorial ceremonies are conducted in May of each year.
In case of the Titan submersible which was lost on June 18th, 2023 when attempting to visit the
wreck of RMS Titanic, the depth at which the implosion happened was nearly 10 times more than
that of ARA San Juan, meaning the water pressure was 10 times more at the time of the accident. We
will never know what the crew of 5 onboard Titan went through in their last moments,
but it is entirely possible that their final thoughts were joyful and exhilarating,
and not tainted at all by the horror of what was about to happen a few milliseconds later.