The sub went missing while carrying five people to the wreckage of the Titanic.

      • I mean, the sub had reached Titanic several times, right?

        So even without the design documents, we know it was previously capable of operating at depth.

        Which we means we know the hull wasn’t made of cotton candy, we know it wasn’t propelled under water by an internal combustion engine, and we know it wasn’t controlled by a device that stops working in water.