300 feet ain’t 12,000.
One engineer pointed out that the extreme compression of the air inside would raise the temperature to thousands of degrees. (Boyle’s Law, you know.) The bodies would flash-burn. (A wag called it dieseling.)
Look!
I learned another new thing today!
That’s 2 in the last 10 minutes!
This Boyle’s Law compression heating scenario is not exactly correct.
Yes, the volume of air inside will be heated as stated.
BUT the volume of this heated air after this compression will be reduced drastically.
The interior volume of the Titan was roughly 1000Ft^3 (The air volume was 1000Ft^3 less the volume of the five inhabitants, which we shall ignore for this calculation).
The water pressure at 12,500’ would be about 400 bar (400 atmospheres).
The 1000ft^3 air volume would have been compressed to about 2.5ft^3, and violently mixed with the 1000ft^3 of seawater that intruded in milliseconds into the craft. Deep seawater has an average temperature of about 4C / 39F. And water has a great heat carrying capacity.
There would have been no noticeable heating of anything by the violently turbulent mixing of the compressed air with the frigid seawater that filled the remaining parts of the craft.