needing materials that would not be required to withstand immense pressure say at 660 ft, a straight wall structure would not be able to withstand a 19atm difference without buckling. A series of smaller differences would make for less costly materials with less precision in their construction...
Ok I am almost positive this doesn't work or it would be used in deep submersibles, but I am at a loss as to why.
About every 33 or 34 feet pressure underwater increases by 1 atm. So a submarine at 33 feet under water with 1 atm internal pressure would have 2 atm of external pressure...