Part of me has been wondering if the noises being heard might be an unexpected discovery of a military (Russian, Chinese, US) submarine operating in the general area. Don’t they submerge for long periods of time without comms?
Typhoon-class: Test depth 900 m (3,000 ft)
Astute-class: Over 300 m (984 ft 3 in)
Akula-class: 480 m (1,570 ft) test depth for Akula I and Akula I Improved, 520 m (1,710 ft) for Akula II and III, 600 m (2,000 ft) maximum operating depth
Ohio-class: Test depth >240 m (800+ ft)
Virginia-class: Test depth >240 m (800+ ft)
Borei-class: Test depth 950 m
Rubis-class: Test depth 350 m
Barracuda-class: Test depth >350 m
Very true, it is a known factoid that submarines sometimes bump into each other because they can't tell that another sub is there. Doing so much noise as to be spotted from aircraft is crazy.
Russia has a Losharik nuclear submarine that was tested up to 2500m (8200ft) if Wikipedia is to be believed. It got in the news when it had a fire that killed half a dozen decorated officers.
USA probably has something similar.
No doubt, depth capability is one of the big features you go after when building a combat submarine. The deeper you go, the faster you can go while retaining stealth (cavitation is the limiting factor which decreases with the added hydrostatic pressure), and, if you can go significantly deeper than your adversaries, you're more or less invulnerable to attack because they can't shoot at you.
That's also why military specs on things like this are always lies, you don't want anyone knowing how deep you can actually go, which likely includes most of the crew.
I think I read in one of the earlier Titan threads this week that density increases pretty slowly with water depth. Water isn't very compressible, after all. So, to your question, I would guess "yes, but not by much".
'just below the surface' -> already at very impressive pressures and given the complex shape of their hulls absolute marvels of engineering. 240 meters below isn't 'just below the surface', that's pretty deep. That the ocean is much deeper is a simple fact but that should not diminish your appreciation for the differences between a 'regular' submarine and a deep sea vessel. Which is also why the latter tend to be extremely simple geometric shapes for the pressure vessel because of the stresses they are subjected to. Ball shape preferred, then cylinder. Something with angles in it would fail for sure.
Each ~30 feet or 10 meters adds an extra atmosphere of pressure. Recreation divers go to 120-130 feet (~4 extra atmospheres). Commercial divers go to ~1000 feet (~30 extra atmospheres). There are rumors that Navi divers can go to ~2000 feet (~60 extra atmospheres).
Not sure why somebody downvoted this. 'Just below the surface' is an odd phrasing, but I had the exact same sort of initial reaction as I think anybody would. The numbers are just so shocking here. The 'average' military sub isn't fit for travel much beyond 300 meters. The peak theoretic technology has subs that may be about to travel to around the 2000 meter mark.
The Titanic is at 3800 meters. That it's reachable, by private organizations no less, is just a marvel.
