I Will Ask You Questions - Now with MORE fantasy ecology!

Imagine that we were horribly wrong about the position of the mantle, and there is actually an 'undersea' below the ocean floor, with various cavernous openings into the seas. This undersea is occupied by a race of sentient cephalopods who are just as ignorant of us as we are of them, but have progressed to a technological level that is, on average, around fifty years behind ours. What would their first impressions of us be, and what barriers would exist between our interactions with them, aside from distance, pressure, and other environmental constraints?

Alrighty, so some background/definition stuff to consider first, just so we've got a handle on exactly what kind of squid-sentients we're talking about. Cephalopods come in two varieties; those with external shells (nautiluses and also the extinct ammonites) and soft-bodied ones like squids, cuttlefish and octopi. Nautiluses can only survive to 800m, while the Vampire Squid is a free-swimmer that probably doesn't go deeper than 1000m. The best example we have of an abyssal cephalopod is probably the Bigfin squid, observed at depths of up to 4700m. Interestingly, both vampires and bigfins are the ones that are likeliest to not be active predators (the shallower-water octopi or predators like Humboldts are almost certainly more intelligent, following the prevailing hypothesis that vamps and bigfins are detritivores/passive scavengers), and neither have particularly acute vision (which is unnecessary in a lightless environment). Most abyssal creatures can detect flashes of bioluminescene consistent with predators/prey, but have little need for more advanced vision in their habitat.

My initial answer would have been "abyssal zones are not conducive to advanced intelligence", but that's only thinking in terms of our intelligence (which was (before tool use) necessary neural wetwork for navigating a 3D space (arboreal) using chiefly binocular vision). Most intelligent life on earth though are generally social creatures and/or actively predatory, which again requires superior sensory equipment.

In keeping with that, what's to see in the deepest, darkest depths of the ocean floor (or cavernous systems in the earth's crust, as the original question posited)? Because the abyssal seafloor ecosystem (at least, the one that doesn't rely on marine snow as a food source) has chemosynthetic bacteria clustered around hydrothermal vents as the base of the food web, thermal vision would be a lot more useful in locating food than photosensitive eye-receptors. But! This forgets that shallower-water, benthic octopodes have many, many nerves in their arms, which could render eyes/eyespots in the body region even more useless. If a species like this became social, they'd almost certainly communicate through touch, possibly augmenting it with chemical cues so you know there's another squid-feller in the vicinity before you get your tentacles all up in their grill. Octopodes can operate their arms independently, so "talking" to someone with one hand while exploring the environment with another wouldn't be weird at all.

In the cave systems (where marine snow won't fall), all your food will be localised around the hydrothermal vents, so learning to harvest it/thin it/etc in a sustainable manner would probably be the earliest attempts at agriculture. This could eventually shift toward better understanding of how hydrothermal activity can happen; maybe even tapping of the seafloor to create suitable "farm"land. There's potential for alternative methods of living if you live closer to an oceanic entranceway and venture out to find food/collect marine snow. Here you'd probably also get technological advancement via various human vessels sinking; these could potentially serve as a kind of divine inspiration for the squidfolk.

I wouldn't see much impetus though for them to try and explore upward; if their main sense is touch I'd imagine they'd be quite an agoraphobic race because having nothing to cling to would be really disorienting. They'd also need technological solutions to navigating the oxygen minimum zone and dealing with lowering water pressure as they ascend up the water column; most conventional solutions would entail sealing them up in a pressurised suit which would severely restrict their senses. I'm struggling to figure how they'd advance to a 20th-century technological stage, but that would actually put us at the right point for them to attempt a "space race", wherein we could have a proper "first encounter."

Assuming, perhaps, that we've posited the potential existence of an undersea race through deep-sea explorations with bathyspheres and submersibles like the Deepsea Challenger, we hopefully won't kill whoever is inside whatever pod manages to breach the surface. The trouble with communicating with an entity like this is a) their fundamentally different perception of the world, because we communicate through audiovisual cues and they use heat+touch+maybe smell? b) they'll need to be kept in a probably-enclosed space to maintain adequate pressure, which further limits their ability to process information from us. Their exploration suit wouldn't have cameras, it'd likelier have haptic noodles. What even are haptic noodles? I sure don't know. First encounters would probably entail a lot of touchy-feely stuff, which I could certainly see humans misinterpreting.

If we actually managed to exchange ideas, though, we could probably share any of our culture that doesn't have a visual component. For an absolute spitball, I'd say they'd be into dubstep because the vibrations from a sufficiently large bass amp would be more detectable than, say, an orchestra (because sound carries a lot better in water than in air).

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If anyone else wants to speculate on how these guys' technological advancement would proceed given the environment they evolved in, by all means pick up this question! I couldn't quite crack it when thinking of how you'd get 50's-era krakens up ins.