What are pressure hulls on a submarine?

Submarines have 2 or more hulls - the superstructure, which is the visible external hull you see, and the inner pressure hull, which contains all the primary equipment and houses the crew.

The space between the superstructure and the pressure hull is where the main ballast tanks are located, as well as line lockers, access/weapons hatches, etc. The pressure hull is where the crew lives and works (and keeps on working...). The primary reason there are 2 hulls is that the pressure hull, while optimally designed for maximum sea pressure resistance, isn't designed for optimum speed and maneuverability underwater. The superstructure, built over the pressure hull, streamlines the boat and compensates for this, making modern boats extremely fast and maneuverable underwater.

The best analogy is a thermos bottle, which has an outer shell (the superstructure) that protects and insulates the inner thermal bottle (the pressure hull). The Russians are known to use a double-layered superstructure on their larger boats, the theory being to insulate the pressure hull from a torpedo attack. The problem with that theory is that modern torpedoes don't need much proximity to severely damage or sink submarine, and any significant damage