And composite materials are basically threads embedded in glue. Threads can be extremely strong in tension. In compression, you have only the strength of the gl

And composite materials are basically threads embedded in glue. Threads can be extremely strong in tension. In compression, you have only the strength of the glue and the fiber/glue interface strength, which isn't a whole lot. Composite materials are in general poor in compression. Crewed submersible hulls are always in compressive stress from the ocean outside. I can't fathom why somebody would choose a composite for a deep submersible hull. It's just asking for a buckling failure. Bad design choice, IMO.