Why are pressure differences so deadly?

Question

The question comes from this video https://www.youtube.com/watch?v=2w-U5wJafhg which details an incident that occurred at the Byford Dolphin Rig https://en.wikipedia.org/wiki/Byford_Dolphin

A group of divers is ascending from the depths of the water. The diving bell they were in had a pressure of 9 atm to help withstand the water pressure while underwater. The diving bell now needs to readjust to the normal surface pressure of 1 atm. However, a mistake was made, and the diving bell experienced explosive decompression (it went from 9 atm to 1 atm super-fast) https://en.wikipedia.org/wiki/Uncontrolled_decompression

One of the divers closest to the opening was pushed through a 2-foot wide crescent shaped opening by the immense force due to the 9 atm to 1 atm pressure difference and was pulverized almost completely. I find this hard to believe. 9 atm to 1 atm is a pressure difference of 8 atm. However, according to this article on deep diving https://en.wikipedia.org/wiki/Deep_diving , divers are exposed to 50 atm of pressure, yet manage to survive. How is it possible that an entire human could be pushed through a small opening with such force that they are destroyed completely with only 8 atm pressure difference?

Answer 1

$8$ atm is $8\times 14.7 \approx 117.6$ psi. Assuming for simplicity a circular opening with a diameter of $1$ ft, the area of the opening would be about $113$ in$^2$, which would correspond to a total force of $113.1 \times 117.6 \approx 13,300$ lbs, which means that our very unfortunate diver would be pushed through that hole with a force equal to the maximum weight of a fully grown African bush elephant.

However, according to this article on deep diving, divers are exposed to 50 atm of pressure, yet manage to survive.

That's because the pressure experienced by divers is being applied evenly to every part of their body. This is a very different thing from having pressure applied unevenly, e.g. so as to cram you through a small opening. Increased absolute pressure of the type experienced by divers can still be dangerous - it affects the solubility of various gases in the blood, for instance, which is why rapid decompression can be fatal. However, having a differential pressure applied unevenly across your body will cause immense structural damage as the different parts of your body are subjected to different stresses.

Answer 2

What J. Murray said. But the pressure forces can be greater than that when water moves and stops suddenly. And underwater shock waves are deadly

The H. L. Hunley was a civil war submarine. It was the first submarine to sink an enemy vessel. It attached a torpedo the the ship and moved away. The entire crew was killed shortly afterward. People have speculated that the shock wave created by the blast was the cause.

Another example is cavitation on a ship's propeller. As the propeller turns, high pressure is created on the front surface, which propels the ship. However, there is low pressure on the back surface. It can be so low that dissolved gasses come out of solution and form bubbles. Something like a soft drink or boiling water. The bubbles collapse, and water rushes in to fill the space. The water slams into the propeller surface so hard that it can erode the surface. See Cavitation Erosion