"One of the last remaining and persistent mysteries of the Pearl Harbor attack is that of the Japanese Midget Submarines. It is a fascinating story of innovation, courage, secrets, and failed expectations. And it is not only a story of the morning hours of December 7, but of the years before to develop these weapons and the years after, where they were deployed in the great Pacific War and how they fared as weapons of war." These words by Daniel J. Basta, Director of the Office of National Marine Sanctuaries, from the foreword of this manuscript, capture both the essence and the impact of this work, assembled by James P. Delgado and his coauthors. Th e authors have combed the records of the Imperial Japanese Navy and the recollections of its veterans as well as US Department of Defense archives. They have logged hours of direct observation and research on the mini-subs in their final resting places, in some cases more than 1,000 feet below the surface of the Pacific. And in the end, they have woven a tapestry of scholarship, historical sleuthing, scientific insight, and good storytelling that will enthrall specialists and history buff s alike.