[Hunpecked]

The 100m depth figure for the Sen Toku subs is also reported in Black_Knyght's original reference. Given the unique requirement of pressure-proofing an aircraft hangar and its large hatch(es), 100m seems quite plausible. As for the subs being "beyond the capabilities of American technology", that's probably true in the same sense that the US B-29 was "beyond" Japan's technology, i.e. technically feasible, but requiring considerable investment, development, and operational experience.

I'm skeptical of the claim that the subs were developed to deliver "small" atomic bombs to mainland US targets. Submarine aircraft carriers weren't unusual in the Japanese Navy (41 such subs total); the Sen Toku class seems like a logical continuation of the concept. More importantly, the Japanese were nowhere near a working nuclear weapon. They hadn't even achieved a sustained nuclear chain reaction, a prerequisite that the Americans achieved some three years before their first nuclear test.