The sinking of a top-secret Soviet nuclear submarine off the coast of Norway has reminded the world that the superpowers are playing a high-technology cat and mouse game beneath the oceans - with high stakes.
The events surrounding the April 7 disaster sound like the plot of a thriller novel, but the loss of a key experimental submarine is very real for Moscow.Norwegian Defense Minister Johan Joergen Holst said that the accident, in which 42 sailors died after a fire and explosion on board, was a reminder of "military realities."
"There is no question - submarines would play a vital role if there were ever a war between East and West," said Rear Admiral Wolfgang Brost, chief of staff at NATO's northern European headquarters in Oslo.
Because they form a key component of superpower military planning, submarines carry some of the most secret and sensitive technology available and there is keen competition to win the advantage that could make a difference in time of war.
"Generally, the United States has stayed ahead," said Richard Fieldhouse, a naval expert with the Stockholm International Peace Research Institute. "But the Soviets have been working very hard for a very long time to close the technology gap."
Although the Soviet Union has the world's biggest underwater fleet, the United States, Britain, China and France all have nuclear submarines.
One of the key strategic areas for submarine operations is off Norway, where the Soviet Mike sank.
In time of war, the Soviet northern fleet would try to disrupt the Atlantic shipping lanes that link the United States with its European allies.
The United States, keen to ensure that the Soviets could be bottled up in the Barents Sea near their main bases, carries out regular naval exercises off Norway and is thought to maintain a major submarine presence there.
The Russians, too, are playing the game in that area.
On one side are the sub-hunters.
Satellites and listening stations monitor the other side's radio and signals traffic. In the air, spy planes cruise in search of submarines.
The hunters also use sonar and hydrophones, mounted on ships, floating or on the seabed.
They can either just listen, sending back the noises to computers that will help pick out the sound of a submarine, or they can send out "active" beams that bounce back off large objects and tell the operator what is under the water.
But the hunted have a few tricks, too.
"Some submarines have little drones, like torpedoes, that they launch when threatened," Fieldhouse said. "These drones have a digital tape which gives off exactly the same sound as the submarine and misleads the hunters."
A skilled sub commander will also know about water temperatures at different ocean levels and how the salt content of seawater varies. These differences can distort sonar readings and be used to camouflage the submarine.