In an interview with Stockton Rush, and sure we should take his insights with a grain of salt, he stated that in deep sea submersibles it's not about weight, bu

In an interview with Stockton Rush, and sure we should take his insights with a grain of salt, he stated that in deep sea submersibles it's not about weight, but buoyancy. The carbon fiber is significantly more buoyant than titanium, meaning they didn't need very special foam and ways to mount it like is used in Alvin.