A South Korean military diver searching for survivors from a navy ship that sank off North Korea has died. The diver lost consciousness as he searched wreckage of the Cheonan warship which sank after an explosion on Friday. <br/><br/>Rescuers have been working to get inside the ship, split in two by the blast. Forty-six sailors are missing. The cause of the explosion is not yet clear, but a senior official has said it could have been a North Korean mine. <br/><br/>South Korean President Lee Myung-bak on Tuesday visited the scene of the wreck flying in by helicopter to Baengnyeong island near the disputed inter-Korean border. He also ordered the military onto alert and said since the sinking took place at the front line, the military should thoroughly prepare for any move by North Korea.<br/>