World's Smallest Tugboat

From Popular Mechanics:
…From prow to stern, the baby boat measures in at just 30 microns, which is about one-third of the thickness of a strand of human hair. In theory, then, the little tug could travel along one of your locks—and you probably wouldn’t even notice. It’s so minuscule, in fact, that you can’t even see this thing with the naked eye…

Maybe this one??: