As previously-noted, the hull is a thin-walled shell under uniform pressure, which increases with increasing depth. Leaks and eventual "failure" of parts of the hull, however one chooses to define 'failure", will start at locations where the geometry of the hull changes, such as around hatch-covers, torpedo-tube doors, seals for propeller-shafts and periscopes, etc. In the case of welded hulls, such as were used by the germans and americans throughout WWII, and by the british in the latter part of the war, two generic causes of 'failure" are defects in and around welds introduced by poor workmanship (pores, cracks, and geometric anomalies at the surfaces of the weld), and so-called "metallurgical notches", which are brittle regions in the weld and surrounding near-by plate (the "heat-affected zone"), which is caused by interaction of the steel being welded with the temperature-cycle of the weld producing hard and brittle metallurgical phase-mixtures in these regions. The first generic cause can be minimized by good design-practice, use of trained welders, careful non-destructive inspection after welding, and careful repair of defective regions in and around welds. The second generic cause can be minimzed by optimizing the weld-process variables (curent, voltage, and welding-speed), in an attempt to produce desirable metallurgical phase-mixtures in the welds and heat-afected zone. If anyone is intrested in additional information on any of these points, please contact me directly at the e-mail address included with thiese comments.