Creating a prison on the high-pressure ice of a deep oceanic planet

Question

I have a superoceanic (H2O) planet ~9.1x Earth’s mass and ~2.1x Earth’s radius. It is a superoceanic planet, meaning the planet is 100% covered in a global ocean 100s of kilometres deep; so deep that at the bottom there is a layer of ice VI (pressure at the bottom = ~10,000 bars). The oceans look turquoise in colour because of ferrous-ferric hydroxy salts dissolved in the oceans. The planet is modelled on current estimated parameters of Kepler-22b.

The atmosphere of this world has a pressure of 10 bars and is mainly composed of Nitrogen and some Carbon Dioxide; quite like Archean Earth but more pressurised. The average temperature at the surface of the water is 30°C.

I want to have a large underwater high security prison for the worst galactic criminals possible on this world. It needs to be set at the very bottom of the ocean floor, on the ice VI layer.

What materials would I need to design this prison of, so that I could maintain the structural integrity within the exterior, and a habitable pressure within the interior of the prison structure? Are there any materials we know of that would make this prison facility believable?

Answer 1

Pressure equilibrium vs material strength (frame challenge)

You do not need a material that is able to withstand the full pressure of kilometers worth of water above an underwater base.

All you need is the same pressure inside that is also coming from the outside - at least in terms of stability.

Meaning: You can resort to average/commonly known materials like aluminum, steel, titanium etc. - what you see fit. But you need to focus on how to get the pressure equilibrium inside it to work. There can be handwaving solutions. Maybe there can be real feasible solutions.

Dome structure

Structurally you would probably aim for a dome with a two hull concept like in a submarine. The dome helps distribute pressure around the structure. The outer hull has only the use of compensating the pressure to the outside water. The inner would be the habitable hull providing breathable air and suitable atmospheric conditions.

The outer shell would probably still need to be reinforced further (in terms of armor) to withstand impacts from large animals, comets, weapon fire, etc.

Onioned Dome

Maybe you can even consider an onion layered dome concept. The outermost layer has to be the strongest, so you have a layer within it that applies counter pressure in a way (close to a full equilibrium) that the material strength is totally sufficient to withstand the forces. This is continued inwards until you can use lighter material strengths to provide the hull for the atmospheric conditions to live in.

Further considerations

What's also needed is a concept how to enter and exit the dome e.g. by docking a submarine? Or teleportation?

Certainly there should also be a concept for energy harnessing deep underwater and how to clean the atmosphere and get rid of waste products.

Enhancing the security level

You can enhance the concept to have something like a "death star underwater". A spheric vessel able to submerge to the ground and also relocate to undisclosed positions, making it even harder for attempted escapes. Instead of someone from the outside "just" needs to dive there, it is now additionally hard to even detect where that prison vessel is anyways... needle in a haystack effect for increased security.