Agatha Christie was born in 1890. She published her first book, “The Mysterious Affair at Styles,” at age 30. Her last one, “Sleeping Murder,” hit bookstores a few months after her death in 1976. She published an astounding 66 mystery/detective novels — that’s more than one per year, starting from that first novel in 1921 — and is one of the best-selling novelists of all time.

All of her mystery novels tell the reader, eventually, who did what and why. But one of the mysteries surrounding Dame Christie — a strange disappearance — has gone unsolved to this day. You won’t find it in one of her novels, though, because it wasn’t something of fiction. Nor was it something that happened to a character she created. It really happened, and the person who disappeared was Christie herself.

In 1926, Christie’s mother Clara Miller passed away. Agatha had to clear out her mother’s home, but Christie’s husband, Archie, apparently wanting to steer clear of illness and death, refused to join her. Shortly thereafter, Archie asked for a divorce — he had taken a mistress whom he wanted to marry. Agatha did not take well to this request, of course, and convinced him (after a short separation) to move back in with hopes of reconciliation. That attempt failed when, on December 3, 1926, the two argued and ultimately, Archie left to spend the weekend with his mistress. Agatha, too, left the house at 9:45 that night. She left a note for her secretary that she’d be in Yorkshire, a county in northern England.

The next day, Agatha Christie’s car was found in Surrey, to the south, roughly 400 km (250 miles) away from Yorkshire. The car contained some of her clothes and her (expired) driver’s license, but no other trace of the mystery writer was to be found. She was gone, her fate unknown. Even in an era before the 24-hour news cycle and Twitter, word of Christie’s disappearance spread throughout the nation and the world — the news made the front page of the New York Times.

Thousands of police officers and volunteers scoured the region looking for the novelist, but to no avail. Fellow mystery novelist Sir Arthur Conan Doyle (of Sherlock Holmes fame) hired a medium (!) to use one of Christie’s gloves in hopes that supernatural spirits would held find the missing lady. But not matter how thoroughly England and its people scoured Surrey or the afterlife, Christie was nowhere to be found.

That’s probably because she wasn’t there. Agatha Christie was very much alive, at a hotel in Yorkshire just like her note had suggested. She had checked in under a false name — that of her husband’s mistress — and emerged ten days after news of her disappearance first broke, and only because someone else at the spa recognized her (and claimed the reward).

Despite the national furor over this mess, Christie and her husband (who did ultimately divorce) eschewed the public spotlight after her re-emergence, without any immediate explanation as to why she turned herself into a ten day mystery miniseries. Many others floated their own theories. Some thought that she was hoping to drum up publicity for her most recent novel, perhaps. Others believed that she wanted people to think she was dead so that investigators would charge her husband with the crime. One more recent explanation claims that she was in what is now known as a “fugue state,” a “period of out-of-body amnesia induced by stress.”

Any of these explanations — and a cadre of others — are possible, but the world will never know. Christie took the reasons to her grave — even her autobiography is silent as to what happened that December.

Bonus Fact : Sir Arthur Conan Doyle’s interest in spiritualism — the use of mediums to speak with the dead — was not a passing phase, and in a comical sense, it cost him a friendship with magician Harry Houdini. Houdini was an outspoken critic of spiritualism, leading to a rift between the two. When Houdini showed Doyle that he could reproduce the so-called mystic powers of mediums, Doyle was similarly unconvinced, and instead, believed that Houdini himself was a psychic. This type of self-delusion isn’t all that uncommon, either, even among people who we’d typically hope could think more critically than the typical person. Another example? Former U.S. Senator Claiborne Pell was, like Doyle, a believer in such nonsense, but when magician James Randi showed him that these types were just using parlor tricks, Pell concluded that “Randi may be a psychic but doesn’t know it.”

From the Archives: The Bourne Identity: More on “fugue states.”

Related: Agatha Christie’s autobiography — her self-told life story, except for one very interesting part.