The loss of the ARA San Juanstunned the international submarine community. It was the first sub lost since Russia’s nuclear-powered Kursk went down in 2000, and it demonstrated the rare international unity and cooperation that occurs when a boat is in distress. At least 13 countries participated in the search for the diesel-electric San Juan — helping the Argentines look for what is now a tomb for 44 submariners who’d gone out for a routine peacetime patrol. Every accident is tragic, and every one offers lessons — lessons that can help captains and crews and engineers avoid the next one, or survive it. The U.S. Navy places great emphasis on submarine rescue, and though their services are rarely utilized, the men and women of the Undersea Rescue Command, based in San Diego, stand ready 24 hours a day to respond to crises.