A one inch, hairline crack, was found on the pressure hull of the 14 year old USS Toledo (a Los Angeles class SSN, or nuclear attack submarine). The crack was in the metal plate, not a weld. After carefully examining the crack, the plate will be replaced. Above the crack, there was a 21 inch hairline crack in the outer (non pressurized) hull, which was under the sail. The USS Toledo had just undergone a three year refit, costing $179 million. The sub will be sent to a nearby (to New London, Connecticut) shipyard, for the repairs.
Such a crack in the pressure hull is a serious problem, because it makes it more likely that the pressure hull would fail, and flood the boat, at less than the "test depth" (about two thirds the "design depth," which is the maximum depth the sub can operate at). Going a little deeper gets you to the collapse (or "crush") depth, at which the pressure hull is crushed and implodes. The deepest diving U.S. subs, the Seawolf class, are believed to have a test depth of 490 meters (1,600 feet) and a collapse depth of 730 meters (2,400 feet). During World War II, collapse depths were never more than a thousand feet. Since then, larger boats, built of stronger metals, have greatly increased the depth subs can operate at. But that only works if the crew knows the limits of their boats, and cracks in the pressure hull reduce those limits.