North Korean submarines
Hoping to locate two North Korean submarines, South Korea has completed a huge naval operation:
South Korea has admitted it has conducted a massive but secret naval operation to find two North Korean submarines.
The Defence Ministry says the hunt began on Sunday after the United States warned Seoul that two North Korean submarines were lurking in South Korean waters.
Surveillance planes and helicopters were dispatched, navy ships dropped depth charges but the operation failed to find any evidence of an incursion.
At the same time, four sailors from the South Korean navy are missing after their semi-submersible boat capsized.
They were participating in what the navy is calling an "anti-infiltration exercise".
Officials say the drill had nothing to do with the submarine hunt.