The sub wasn't steel, it was a carbon fibre weave. The tiniest crack would have caused catastrophic shattering of the hull.

For every ten meters of descent, pressure increases by 1 atmosphere. At 4000 meters, that's 400 atm, or ~40 Mega Pascals, which is significantly higher than the pressure caused by the bomb that leveled a building in Oklahoma.

The hull shattered catastrophically, and the pressure wave liquified the occupants instantly. It's horrifying, but at least it was painless.

Yup - and if someone had been inside the tank in that YouTube video they’d likely have been dead almost instantly too when it imploded. (Even if we forget about the vacuum)

Probably the fastest and least painful way to die, short of sitting on a 2000 lb bomb or the like when it goes off.

Interesting a commenter on the video notes the pressure there is equivalent to only 10 meters underwater. Imagine the force 2 miles below

I am probably too stupid, but how is that possible? A lot of divers go 10 meters underwater and don't die of pressure, right?

Pressure equalizes as they descend slowly. It's different from descending 4000m inside an armoured can and have it all released upon you in a split second.

Humans are less compressible than an empty steel tank (being mostly water) and when they dive they don't teleport from 0 to 10 meters in an instant.

Well, sea level is at 1 atm already, so an increase to 2 atm isn't immediately catastrophic. However, if you took that tank filled entirely with sea level air and shoved it down 30 feet under the water the same thing would happen. It instantly crumples because its hollow. We're full of water with some air in a couple places. We'd crumple too if we were just a layer of skin over an air sac. But as it stands, the human body can withstand that pressure alright

10 meters of water is one atmosphere, so they're right - 0 on the inside and 1 on the outside is a pressure differential of one atmosphere.

"almost 4000 meters of water" is "almost 400 atmospheres". About 2.7 tons per square inch. I find that that absolutely unimaginable. - I can't find any equivalence that my brain can make sense of.