Submarine

Submarines are definitely a known technology in Agatha's Europa.

The Comic

A climatic scene in Agatha's favorite Heterodyne story, The Heterodyne Boys and the Race to the West Pole, revolves around Bill and Lucrezia having an.. intense.. conversation on board a distressed submarine. In another story, The Turbines of Atlantis, Bill and Barry are menaced by a submarine fashioned in the shape of a giant shark-like creature.

Agatha herself doesn't personally encounter this mode of transport until the course of events leads to her traveling to England. That country's increasingly-submerged state means its citizens make extensive use of the vehicles in and around her domed cities, as was foreshadowed as far back as the Secret Blueprints. Agatha and Co. travel to England in an airship, but then attempt to intercept the arriving Madwa Korvel and Dio Zardeliv's submarine, which comes crashing into an airlock at the city docks with everybody on board dead. Our Heroes then travel to and from the undersea dome of the Queen's Society in subs, the chaotic return journey being interrupted/saved by a Great Cetacean.

England also deploys as border-guards less traditional undersea vessels shaped like giant sea-serpents. Gil and Bang hijack one of these as they chase after the kidnapped Tarvek.

The Novels

At the end of the second print novel, Wooster broaches the idea of taking both Agatha and the members of Master Payne's Circus of Adventure from Paris to Londinium on board a submarine. Presumably the latter trip happens, but if so, it occurs off-screen.

The Works

A number of cards in the original version of The Works are for submarines, equal in fact to the number of airships. Furthermore, scoring some of each kind is a shortcut to victory. Although The Works is not styled after military action, it gives one the feeling that an eventual conflict between the Air (The Baron's Peace) and the Sea (England) for World Domination could be in the cards.

Possibly relevant outside information

Captain Nemo's vessel the Nautilus is of course the great-granddaddy of all steampunk-type submarines, but the esthetics of the bulk of England's submarine fleet is openly modeled after the eponymous vessel in the animated film Yellow Submarine.