The huge Japanese sub is likely an 1-400, a Sentoku class submarine. At 400 feet in length, it was the largest sub in existence until nuclear powered ballistic submarines were built in the 1960s. In comparison, a type VII U-boat was 220 feet long. Only 3 Sentoku class subs were ever built. They could carry 3 small aircraft and had a range of over 43,000 miles.