I think it is from here:

Catastrophic Implosion of a submersible explained:
When a submarine hull collapses, it moves inward at about 1,500 miles per hour - that’s 2,200 feet per second.
The time required for complete collapse is 20 / 2,200 seconds = about 1 millisecond.
A human brain responds instinctually to stimulus at about 25 milliseconds. Human rational response (sense→reason→act) is at best 150 milliseconds.
The air inside a sub has a fairly high concentration of hydrocarbon vapors.
When the hull collapses it behaves like a very large piston on a very large Diesel engine.
The air auto-ignites and an explosion follows the initial rapid implosion. Large blobs of fat (that would be humans) incinerate and are turned to ash and dust quicker than you can blink your eye.
Info Source: Dave Corley, former Nuke sub officer

https://twitter.com/olilondontv/status/1671951053753909255?s=61&t=tYcx9K8IT1b1jHEqELZ4Vg

unless of course there was some creaking and cracking noises first, then maybe a little spray of water.

Glad they didn't have to suffer