An unmanned semi-submersible vehicle may prove that the best way to travel in water undetected and efficiently is not on top, or below, but in-between. The roughly 1.5-foot-long semi-sub prototype, built with off-the-shelf and 3D-printed parts, showed its seaworthiness in water tests, moving quickly with low drag and a low profile. This vessel-type isn't new. Authorities have discovered crudely made semi-subs being used for illicit purposes in recent years, but the project aims to demonstrate how engineer-developed half-submerged vessels can efficiently serve military, commercial and research purposes.