According to Official Guinness Records,
The three Japanese I-400 Sen Toku class submarines built during the second world war each carried three Aichi M6A1 'Sieran' seaplanes, which were stored folded in a 35 m (115 ft) hangar on the submarines' decks and launched along the deck by catapult. The subs displaced around 6,000 tonnes (13,228,000 lb) submerged and were 122 m (400 ft) long, making them the largest ever at that time.
For a complete list of Unknown date records, please visit Unknown date Guinness Records in Japan.