According to Dick O'Kane's book, "Clear the Bridge" (as I recall off the top of my head here at work) Dick was aware that all subs were designed with crush depths well beyond the 'design' depth they weren't supposed to go beyond. Knowing that depth could mean survival in an evasive maneuver, he wanted to know how deep the Tang (the first thick skinned Gato class boat) really could go.

so he took some Electric boat engineers out to lunch, got them properly lubricated with alcohol, and starting pumping them for the truth. He eventually got it out of them that it should go well past 600ft as far as crush depth, but many of the fittings and joints might give way or leak before then.

So he took the Tang out, dove it down till something started to leak, came up, devised a fix for it, went out again, dove deeper till something else leaked, fixed that, and so on till he could realiably and securely get the boat well past 500ft.

This greater than rated depth was used in combat many times. The funny part is he did this testing in secret from his commanders, and Tang's capabilities were not widely known outside of its crew.

In one action in the Pacific the Tang lost depth control, leaving the control room depth gauge pinned past its max reading of 600ft for a LOOONG time. Long afterwards, when they were both POW's, a forward torpedoman told O'Kane that he saw an external pressure gauge in that compartment go past 375psi (750ft depth), but decided not to tell O'Kane lest he decide the boat was safe to such an extreme depth and use it routinely in battle!