Sailor's magazine Marine Buzz has an article about smuggling boats. Smuggling is an area in which insane projects are the norm, because sane ones inevitably get caught.
In the beginning, people wanting to bring over illegal goods did so on foot. Typically, they were caught. This annoyed the kingpin, because his goods were confiscated, and the mule, who got severely punished. (I'm not naive enough to believe that the kingpin gave a hoot about the mule.)
So then, planes. For a little while this worked. Then problems emerged first with dropping the goods in the wrong places, where it hit people. Hitting people with illegal things kind of draws the wrong kind of attention. Also, planes tend to have things like transponders that make them extremely blatant.
So then the idea of ocean-based smuggling occurred. The perpetrators built really narrow boats with a flat, ocean-color-painted top. The boats were then stuffed with guidance electronics, loads and loads of illegal goods, and an operator. The operator would sail the boat to an obscure shore where it would be unloaded and then abandoned. (The boat was apparently cheap enough that a new one could be built every smuggling operation.) The boat sounds no larger than my bathroom in total, but still had hundreds of millions of dollars worth of goods on it.
And for a long time, the boats worked, because they were extremely hard to see. And when they were seen, they were confused with other things, or sunk with the operator escaping, thus destroying all the evidence. Then the coast guard developed a way of capturing these boats, by finding their most common materials via sonar and abruptly inserting a commando before the operator notices.
So the next idea? Goddamned submarines. Professional navies have difficulty finding enemy submarines, and a litany of technologies have been developed to make them even more invisible. The thing runs under complete radio-silence, making at most a whirring noise from the propulsion engines, and there are ways of suppressing that, too. Even this will eventually be caught by the coast guard, and probably whatever they discover will revolutionize submarine warfare forever.