On July 20, 1923, Pancho Villa was gunned down in his car in Parral, Mexico, and that should have been the end of the story. It wasn't. Three years later somebody dug into his grave in the middle of the night, and whatever they took has never surfaced.

The documented part is thin: in 1926 grave robbers opened Villa's tomb. Depending on which account you read they took the whole head, or just the skull, or possibly nothing at all, because when the coffin was exhumed decades later during an official 1976 investigation some officials reported the remains looked intact. Others disagreed. Nobody's account fully matches anybody else's, which for a story like this is basically the fuel.

The theory that refuses to die is the one you've maybe already guessed, because it attaches itself to every missing skull in the Western Hemisphere: Yale's Skull and Bones society. The claim is that the secret society sent someone, or financed someone, or at minimum received someone's delivery, and that Villa's skull has been sitting in a building in New Haven called the Tomb for the better part of a century, allegedly used in the society's rituals alongside the stolen bones of Geronimo, whose grave was also robbed, in 1918, by members with ties to the same club. Two famous skulls, one secret society, two robberies within a decade of each other. It's a tidy shape for a story to take.

And it's a great shape. Revolution, secret societies, a heist, a building with no windows. The problem, always, is that the evidence is soft where it should be hard. The Villa family believed the head was taken, and said so for generations. Skull and Bones has never confirmed anything, which of course proves nothing either way, since a secret society that confirmed things wouldn't be doing its job. Geronimo's descendants sued in 2009 to recover his remains and got essentially nowhere, in part because nobody could establish what was actually in the Tomb. The whole case runs on a chain of maybe.

There are rival versions, and they're worth sitting with. One holds that political enemies mutilated the corpse as revenge, a grimly ordinary motive, and that the grave robbery story grew up later to explain the condition of the remains. Another suggests the theft was simple relic-hunting by people who understood exactly what a Villa skull would be worth to collectors in either country. And the 1976 exhumation, the one chance to settle it, reportedly produced conflicting observations from the people standing right there at the open coffin. I find that's how these things always go. You finally get the box open and the witnesses can't agree on what they're looking at.

The MSN feature on the case walks through the century of theories, and what struck me reading it is how the question has outlived every person who might have answered it. The robbers are dead. The family members who insisted the head was gone are dead. Skull and Bones members from that era are long gone. What's left is a gap in a coffin and a story that fits the gap a little too neatly.

And that's the part I can't stop chewing on. If the skull is in New Haven it's been handled by generations of young men who went on to run banks and governments and newspapers, all of them inducted in a room with a dead revolutionary's head. If it isn't, then a hundred years of folklore grew around an empty accusation, and Villa's skull is wherever skulls go when nobody's looking. I don't know which version is stranger. I know which one I'd bet on, and I know it's the one I can't prove.

The grave in Parral still gets visitors. Whether anyone is buried there in full is, officially, an open question.