The untold story of the Royal Navy's experimental submarine cruiser, built in secret after WWI, and torpedoed by political propaganda. The 'X' stood for experimental, but it might equally have meant extraordinary, exotic or extravagant, as this giant submarine attracted superlatives. Built in the early 1920s, it was the world's largest, most heavily armed, and deepest diving submersible of the day. A controversial project conceived behind the backs of politicians, X.1 would remain an unwanted stepchild. As British diplomats attempted to outlaw the use of submarines as commerce raiders, the Admiralty was building the world's most powerful corsair submarine, designed to destroy entire convoys of merchant ships. This book explores the historical background of submarine cruisers, the personalities involved in X.1's design and service, the spy drama surrounding her launch, the treason trial of a Royal Navy submarine commander, the ship's checkered career, and her political demise. Despite technical successes, the X.1 became the target of a misinformation campaign aimed at persuading foreign naval powers that the cruiser submarine did not work. While the myth of her failure persists even today, it was ignored by other navies, who went on building submarine cruisers of their own. The book analyses the submarine cruisers built by the US, French, and Japanese navies, as well as the projected German copy of X.1, the Type XI U-Boat.