For nearly 50 years, conventional manned deep submersibles have operated on the principle of a free balloon or blimp (powered balloon). Buoyancy is normally provided by a plastic matrix material
filled with glass microspheres, each containing a bubble of air. This cast syntactic foam is easily formed to give needed lift and a streamlined configuration to the vehicle. Small propulsion and thruster motors provide limited maneuverability while submerged and precise control while hovering. Various ballasting systems give positive, neutral, or negative buoyancy during underwater operations.
While seemingly an arcane way of operating, the balloon/blimp design is just right for the slow "flight" requirements for most submerged operations, especially scientific research. The human observers inside the vehicle do not need speed as much as the ability to hover or stop at a desired work site.