Four days after an explosion sank the South Korean ship Cheonan, the search for 46 unaccounted-for marines continued deep in the Yellow Sea Tuesday with divers drilling a hole into the hull. Fifty-eight of the 104 crew members survived, but hope is dwindling fast for the rest. The rescue effort has already claimed the life of a 53-year-old diver who fell unconscious in the choppy waters. The cause of the explosion which broke the ship in two is still a mystery, though a leading theory is that it collided with a North Korean mine left over from the Korean War nearly 60 years ago. Investigators have not ruled out an internal malfunction.