At least one boat went over 1000ft (~310 m) and lived to tell about it.

The maximum \'test\' depth was widely known to be very conservative, and was often violated to escape escorts \'by going deep\'. More often extremely deep dives, well beyond test depth, were made by accident when battle damage caused malfunctions and leaks, making the boat heavy, the loss of depth control.

Sub hulls were generally designed for a collapse depth well in excess of the max rated depth \'test\' depth - I believe the hulls were designed with a safety factor of two - ie: the design collapse depth was twice the rated maximum depth, or \'test\' depth. The actual hull would probably collapse a bit beyond the design crush depth.

The problem with violating the test depth was not that the hull would immediately collapse, but rather that all the fittings (valves, etc.) would begin to leak at extreme depth. If you were already heavy and out of control, this could be quite a problem, especially if it was a serious leak - with several hundred psi of head behind it, even a normally small leak could spout huge amounts of water, and send an already damaged and heavy boat plunging uncontrollably downwards.

Obviously, if one did not arrest an out of control descent, eventually the hull would collapse.