Since the surface of the 22nd century Earth was not safe from the prying eyes any more, the military started seeing the deep ocean's potential as the new No Man's Land, great and unknown. It was already being traveled of course, as many military and civilian vehicles were cruising underwater tending to the young Sea Colonies, but only around 2140 the military doctrine started addressing The Depths as a primary combat environment. Building upon the tradition of the aerial and orbital warfare, the Arabian League engineers proposed a revolutionary concept of a mini-sub that would work in combat-capable groups and dominate the seas, either by outmaneuvering and ousneaking or even directly challenging slow and cumbersome nuclear submarines. With the mastery of Liquid Breathing and new, efficient underwater engines, such a sub could be small, swift, and yet capable of reaching great depths - and, more importantly, rapidly descending or ascending as required. The first mass-produced model was the DX-3 nicknamed "Waterbed", but many more followed soon; the era of underwater dogfight had begun.