From: "Frank Haber" 

A landlubber asks

1. I presume a hatch trunk is the equivalent of an airlock on a space ship.

2. and that both hatches open outwards, and have wheels to dog them down on
the inside, and seal tighter with increasing depth.

So how much does the diameter of a pressure hull shrink at 300m?  How much
does the cover flex?  (I presume it's a nice, ribbed 3/4" or so of
steel.) Does the hatch wheel actually and visibly show more stem as the sub
dives?

And how can they risk the single point of failure by working in the trunk
at depth with the inner hatch open?

--- BBBS/NT v4.01 Flag-5