For educational purposes

During World War Two, Britain was absolutely reliant on maritime trade for its survival and therefore the protection of its merchant fleet became an issue of paramount importance.

After June 1940, however, German U-boats began to operate in the Atlantic and appalling losses were inflicted on allied merchant shipping.

Arctic convoys were twice suspended in 1942 as German aircraft and submarines wrought havoc on the waves - the trip to the Russian port of Murmansk, for instance, became known as the 'Death Trip' because so few ships returned.

For men aboard, there was little hope of escape should a ship go down; they could expect to survive for only a few minutes in the icy waters.

Featuring the filmed memories of convoy veterans this is the story of one of World War Two's most pitiless theatre of operations, where men lived in constant fear of the unseen dangers of the deep.

The programme also uses 3D graphics and animation to show the realities of a submarine attack and the mechanics of a ships' defences. Leading maritime and military experts are also featured.