JAPAN'S SECRET WEAPON

Find out why one of the most powerful tools of the Second World War was never used.

One of the best-kept secrets of the Second World War was a huge aircraft-carrying submarine designed to deliver Japanese bombers to within a few hundred kilometres of American cities.

Engineers and historians conduct experiments and demonstrations to reveal the secrets behind a technological marvel that could have changed the course of the war.

Plus, archive footage shows what made the I-400 subs so deadly: the three state-of-the-art attack bombers it could carry anywhere in the world. But why was the technology never used in an attack on the US?