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The Bulb rescue shuttle

Virtually just a single ballast tank with a nav console and an engine, perhaps the smallest still practical conceivable sub. Top speed over 120 km/h. Too fast to easily navigate.

Diving speed much faster than ascension speed. Have tested it to successfully run campain missions; keep an extra set of diving suits and masks on board so that your escorts don't suffocate. You have to modify the sub file so that it is not classified as a Shuttle.