[Mahler9th]

Perhaps of interest:

https://www.researchgate.net/publication/235174559_Ceramic_External_Pressure_Housings_For_D eep_Sea_Vehicles

"Only glasses, ceramic and carbon fiber reinforced plastic can provide the necessary weight to strength ratio to make the external pressure housings for undersea vehicles positively buoyant at the abyssal design depth. This group of materials poses unique challenges to the designer and fabricator of pressure housings. This paper summarizes the findings of the R & D program aimed at developing the technology for the design, fabrication, and testing of ceramic housings culminating in the application of ceramic housings to the WHOI ROV/AUV diving system with 36,000 ft (11,000 m) capability (Figure 1)."

https://www.researchgate.net/publication/289330840_Operations_to_11000m_Nereus_ceramic_hous ing_design_and_analysis