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Sea Spies (9pm History)

The ocean is known as "the silent world", but that is a misnomer. It is a cacophony of noise underwater, with whales singing, seabed volcanoes erupting and bubbles exploding. Added to that, noise carries a great distance under the surface: a charge exploded underwater can be heard up to 12,000 miles away. This proved a problem during the cold war, when the superpowers spent billions of dollars trying to gain dominance of the oceans by finding ways of cloaking the sound of their submarines. Presented by Robert Ballard, formerly of the US navy, this intriguing film recalls how boffins on both sides came up with increasingly ingenious stealth devices. For example, they invented "burglar alarms" attached to the seabed to detect enemy subs, and "quieting" technologies that could mask the sound of a craft. Ballard characterises the so-called "battle for sea silence" as "an amazing game of chess, requiring incredible knowledge of how the ocean works".