The invention in one variation is a pressure vessel having a pair of hemispheres with a wall and a circular edge, a pair of sealing elements, and an intervening mounting plate with circular groove on both sides of the plate. The hemispheres are mounted on opposite sides of the plate. Each groove has an sealing element and the circular edge. A seal is formed between the plate and a hemisphere when the hemisphere is pressed against the sealing element. Pressure can be generated by creating a partial vacuum in the sealed hemispheres or by using clamps. When exposed to higher pressures, such as when the vessel is used underwater, the pressure on the hemispheres is conveyed to the sealing element forming an even tighter seal. The vessel has interface connections that enable communication to and from devices mounted inside. The vessels are modular and can be ganged to provide redundancy or different functions.