"They were the only thing that really frightened me." So said the usually steadfast Winston Churchill of the German Navy's U-Boat fleet - the dreaded Wolf Pack, which stalked the Atlantic depths, bringing instant death to merchantmen and warship alike. Together with a formidable surface fleet, the U-Boats challenged the Royal Navy's traditional supremacy of the oceans, countering Britain's strength in numbers with a reign of silent terror. A decade of secret rebuilding had brought forth capital ships like the Scharnhorst, Gneisenau and Prinz Eugen - names to strike fear into any Allied seafarer. However, it was from under the water that the biggest threat was to come. Commanded by Admiral Karl Doenitz, the submarines laid waste to shipping, cutting supply lines and striking from within convoys where Allied escorts dare not use guns. The U-Boat captains were the equivalent of Lufwaffe aces - ruthless, cunning, and ice-cool under pressure. Enduring harsh conditions for months on end and working in darkness with no chance of escape, this was truly a war of nerves. "There is no margin for error on a submarine," said one captain. "You are either alive or dead." On May 4 1945, Doenitz issued orders to cease hostilities. "You have fought like lions," he said to his valiant survivors. "You are laying down your arms after a heroic fight which knows no equals." The story of that fight is told here using rare archive footage, much of which has been unavailable in the West since World War II.