BBC
Test depth for the TR-1700 class submarines is 300 meters. While that may be exceeded, by a factor of two (roughly), that calculated crush depth approaching 600m is only theoretical. In wartime, due to flooding or other factors, theoretical crush depths have been exceeded and the vessel surfaced after repairing the problem that put them below crush depth. Some of that area is at depths where the boat is likely to have remained intact, and some where so deep it is unlikely to have retained pressure hull integrity.