In a steampunk world, or historically in general, when could be the earliest technology to dive to the deepest parts of the ocean and what would that technology look like?
To prime the question, some technologies needed:
Buoyancy - provided by lighter than water fluids like gasoline (0.7 specific gravity) or kerosene (0.8) or whale oil (0.9). Possibly by pumice stone? Though would pumice pulverize at depth?
Pressure hull - steel, but specific types of steel needed? Could bronze substitute? If bronze, how much thickness?
Glass - acrylic glass was used, can other glass be used?
Controls - other than a diving bell, if given thrusters/control surfaces, how could these be manipulated from the pressure hull?
I'd say the technology is in the grasp (but a reach) for steampunk/victorian era. But possibly do-able?