To propose a situation that’s famous, but no where near as emotionally charged as many more modern conspiracy theory magnets, take a look at the story of the Marie Celeste.
The short, bare bones version, for those of you who don’t recall the name, the Marie Celeste was a merchant sailing vessel of the nineteenth century. She sailed from New York Harbor to the Med, and then was found near the Straits of Gibraltar, under sail, but with no sign of her crew. She was taken in tow by the crew of the ship that found her, and brought to the British authorities in Gibraltar, where the conspiracy theories began building, and have been going strong ever since.
At the time that the ship was brought to harbor in Gibraltar the proceedings to have the ship claimed for salvage began. Because the Dea Gratis had been moored next to the Marie Celeste one of the officers at the Admiralty Court decided that it had to have been an act of piracy, not misadventure, that dealt with the crew of the Marie Celeste. In the end the Admiralty Court found no evidence of a crime and had to award the salvage to the master of the Dea Gratis. But the Admiralty Court also dropped the award, as punishment for the unnamed and unproven crimes that the Court seems convinced must have happened, anyways.
Since then such lights as A. Conan Doyle, and others have tried to prove what happened to the Marie Celeste. The theories, as the wikipedia page says, range from the plausible, and rational, to the absurd, the insane. The Occult theories actually are some of the less wild theories for the loss of the crew.
It is my opinion that the cause of the loss of the crew of the Marie Celeste, while chilling, and tragic, requires no extraordinary explanations. For whatever reason, the ship showed signs of having been hastily abandoned by the crew. Her cargo holds were opened for maximum ventilation. And one of her small boats was missing as well.
This adds up to a simple scenario: She was shipping industrial alcohol, a cargo that if it started leaking, could build up explosive fumes in her cargo hold. At Gibraltar it was found that one of the casks was broken, and had leaked into the hold. A number of researchers have suggested this is the ultimate cause of the fate of the crew. Having opened the hold for an inspection, the crew assumed that they had a larger problem with potentially explosive fumes building up in the hold than a single breached cask would have caused. To vent the hold, all the accesses were opened, and then the crew debarked to the hastily lowered missing small boat, so they could tie off the small boat, and ride out the ventilation in relative safety away from the ship.
Alas, in such circumstances, it’s easy to believe that the person who was meant to tie off the small boat did a slip-shod job of it. At which point, the small squalls that are common near the Straits of Gibraltar probably came up, and in the resulting blow, the small craft’s painter came undone - and the crew was doomed to watch as their ship sped off, with the wind, while they could do nothing but watch.
It’s a conjecture, of course, since several key points of the theory are based solely upon deduction or suppositions. I think it’s solid, but it’s not something that could be categorically proven. And anyone familiar with sailing operations will find the sequence of events to be horribly possible.
And that still doesn’t stop the theories from being promulgated. Piracy, aliens, The Bermuda Triangle, insurance fraud (made a bit harder to dismiss out of hand, given that a latter owner of the Marie Celeste did try to sink her and her cargo for the insurance money.), disease, suicidal cults, occult worship and others have all been proposed, with apparent sincerity, as explanations.
And, really, the cause of this whole industry is that officer back in Gibraltar who was originally convinced that things couldn’t possibly be as simple as the facts presented in court, which the same court ruled on, could be the truth. It’s the triumph of theory over evidence.