I think I’m misinterpreting what “rated for 4000m” means. My impression is this is the maximum safe depth the sub is capable of reaching without damage. And that they are bring it regularly to 95% of that depth.

My understanding of safety margins, as I was taught, is that one should design such that you have a safety margin of 1.5x the intended use case. In other words, for 3800m expected frequent usage, it should be able to reach a depth of 5700m.

Alvin, for example, can reach a depth of 6500m.

But you’re right, we don’t know what that 4000m means, really. I’m just assuming it means safe depth without damage.