Sure Sphere is a filthy piece of literary swill but it also a vital component of any submersible. As you may remember from geometry, for any given volume, a sphere has the smallest surface area, or for any given surface area, a sphere will have the greatest volume. From a practical standpoint for a deep-sea submersible, this means less surface for pressure to act upon. Thus the choice for the ‘large’ compartment to hold humans on a deep diving submersible is a sphere.
The Bathysphere was made by the Watson Stillman Hydraulic Machinery Company in Roselle, New Jersey. Made of cast iron, it could hold two people. The “walls” measured one and a half feet thick and were made of a single casting of the finest open-hearth steel. The Bathysphere and its cables cost Barton $12,000.
The deepest diving sphere was the one that hung below float filled chamber filled with gasoline. In 1960 the Trieste bathyscaphe descended to 10,900m in the Mariana Trench. Big enough to hold just to people
The pressure sphere was built by the Krupp Steel Works of Essen, Germany, in three finely-machined sections (an equatorial ring and two caps). To withstand the high pressure of 1.25 metric tons per cm² (110 MPa) at the bottom of Challenger Deep, the sphere’s walls were 12.7 cm (5 inches) thick (it was overdesigned to withstand considerably more than the rated pressure). The sphere weighed 13 metric tons in air and 8 in water (giving it an average specific gravity of 13/(13-8) = 2.6 times that of sea water). The float was necessary because the sphere was dense: it was not possible to design a sphere large enough to hold a man which would withstand the necessary pressures, yet also have metal walls thin enough for the sphere to be neutrally-buoyant.
Modern spheres are a whole other ballgame. Harbor Branch Oceanographic Institutes’s submersible contains a solid acrylic sphere allowing scientists exceptional views. These views come with a price as it limits the Johnson-Sea-Link to less relatively shallow depths (>1500m). The more familiar Alvin, soon to be replaced, possesses a titanium sphere. As you can see from the figure below, the sphere rests forward and most of the external shell of the Alvin contains ballast and syntactic foam to regulate buoyancy. The sphere in modern submersibles can be released from the rest of the body in emergency situations to return to the surface. Jim Richardson as a thorough and excellent writeup with pictures at NavSource Online about another Alvin Class submersible the Sea Cliff (DSV-4). Included is some wonderful photos of the entire construction process.