Exploring the Mariana Trench

Consider you are a deep-sea explorer and want to shoot a movie in the Mariana Trench, where your submarine vessel endures immense pressure. At these deep levels, a sub’s hatch alone is subjected to 4 million pounds of force, which is around four 747 planes stacked on each other!
  1. Why might you forgo your stealth military sub and choose a spherical vessel?
  2. Would you prefer your submarine’s hull to be made from Aluminum, Steel, or Titanium? Why?

Hint
For material, factor in strength and corrosion resistance.
Hint 2
For shape, consider the pressure distribution.
At the bottom of the ocean, a huge engineering concern is being crushed under all that pressure. As a designer, you would opt for a spherical shape to evenly distribute the pressure force across the entire surface area. A tube-shaped vessel, commonly found on military designs, does not distribute the force evenly, and will buckle under much higher depths.
In terms of material strength, Steel is the strongest option to handle the huge load on the hull. However, Titanium is the optimal choice due to its corrosion resistance. Salt water would eventually rust away the Aluminum and Steel, compromising the design integrity. Luckily, Titanium does not react with sea water.
As a designer, you would opt for a spherical shape to evenly distribute the pressure force across the entire surface area. Titanium is the optimal choice due to its corrosion resistance.