I'd like my (very well funded) group of scientists to conduct an important excavation, in-situ with fully breathable air and comfort. I realize a pipe down to the surface would require a pump and could cause issues.
What I'd like is a cylinder wide enough that there is natural ventilation, and they can continue their excavations, dry, and breathing naturally without any automated pumps, etc.
I'll handle the depth of the cylinder into the seabed later, and of course this is an expensive prospect, but basically want to have a perfectly breathable seabed at 500m depth. How wide would it need to be?
PS - It's about the elevation of the dead sea.
EDIT: I am looking for an open space that allows for the free-breathing of the excavators. Specifically the width of the air-space (I allow a conical shape of course as well). The structural, safety and financial features are a separate question. This is purely about breathe-ability.