Pressure hulls should be made out of contiguous material like steel, titanium, ceramic or acrylic, he explained, in order to do modeling and finite element analysis to "understand the number of cycles that it can take." That's not the case with a composite material, like carbon fiber, made of two different materials blended together.

"And so we all knew that the danger was delamination and progressive failure over time with microscopic water ingress and ... what they call cycling fatigue," he added. "And we knew if the sub passed its pressure test it wasn't gonna fail on its first dive ... but it's going to fail over time, which is insidious. You don't get that with steel or titanium."