Agatha Christie, the best-selling novelist of all time, is known for her 66 detective novels and 14 short story collections, as well as the world’s longest-running play – The Mousetrap. Her books have sold over a billion copies in the English language and a billion in translation, second only to the Bible in terms of sales.

And yet, despite all her literary mystery success, she is almost as equally well known for her own mystery: an 11 day disappearance in December of 1926 that remains, to this day, unexplained.

Agatha Mary Clarissa Miller was born on 15 September 1890 in Torquay, Devon, South West England into a wealthy middle class family. The youngest of three children, she was home schooled and spent most of her youth alone with her imagination. Because her siblings were much older and away at boarding school, it’s not surprising that the bright and inquisitive young Agatha taught herself to read by the age of five. As equally unsurprising was the birth of aspiring novelist dreams soon thereafter.

At 18, Christie wrote her first short story, “The House of Beauty,” a 6,000 word tale about madness and dreams. More short stories followed, all of them rejected by the magazines to which she submitted. In a style well-known by countless of other authors both before and since, Christie persisted, starting work on her first novel around the same time.

Marriage and World War I interrupted her literary dreams momentarily as her new husband, Archibald “Archie” Christie, was sent first to France and then back to Britain as a colonel in the Air Ministry. Christie herself served as a member of the Voluntary Aid Detachment of the Red Cross, working in the Town Hall Red Cross Hospital, Torquay, first as a nurse then as a dispenser.

Upon her husband’s reassignment to London, Christie again focused her attention on writing, trying her hand at detective novels, which she had always enjoyed. The Mysterious Affair at Styles was rejected by Hodder & Stoughton and Methuen, eventually finding a home at The Bodley Head, which not only published the book in 1920, but also signed her to a contract for five additional novels, all of which saw success.

But, with her growing literary success came pain on the homefront. Despite the birth of a child in 1919 and the luxurious lifestyle Christie’s career entailed, the relationship between Christie and her husband grew strained and, in August 1926, Archie asked his wife for a divorce, announcing he had fallen in love with fellow golfer Nancy Neale. Agatha was devastated.

On 3 December 1926, the pair quarrelled after Archie announced his plan to spend the weekend with friends, unaccompanied by his wife. After Archie stormed out, Agatha Christie got up from her armchair and climbed the stairs of her Berkshire home. She kissed her sleeping daughter Rosalind, aged seven, goodnight and made her way back downstairs again. Then she climbed into her Morris Cowley and drove off into the night. Although her car was discovered the next day at Newlands Corner in Surrey, parked above a chalk quarry with an expired driving license and clothes inside, Christie herself was gone.

She would not be seen again for 11 days.

What followed was one of the largest manhunts in British history. More than a thousand police officers, 15,000 volunteers, and–for the first time ever–even airplanes searched the rural landscape. Two of Britain’s most famous crime writers, Sir Arthur Conan Doyle and Dorothy Sayers even joined in, with Doyle giving a spirit medium one of Christie’s gloves to find her.

And yet still, she remained missing.

Rumors swirled. Since her car was found close to a natural spring known as the Silent Pool, where two young children were reputed to have died, some suggested that Christie had deliberately drowned herself. But the lack of a body squashed this idea before it could gain steam. Others believed the incident to be nothing more than a publicity stunt, a clever ruse to promote her new book. Still others began to point fingers at Archie, theorizing the estranged husband may have killed his wife.

Then, on 14 December 1926, Agatha Christie was located at the Swan Hydropathic Hotel in Harrogate, Yorkshire, 184 miles north of her home, registered as “Mrs Tressa Neele” from Capetown South Africa. She failed to recognize Archie when he arrived and couldn’t–or wouldn’t–give the police any information about how or why she was there. She was whisked away to her sister’s estate in Cheadle where two doctors diagnosed her with “genuine” memory loss.

That wasn’t enough to satiate a ravenous public.

Despite the public outcry for answers, Christie remained in seclusion for several months afterward, eventually granting Archie the requested divorce but refusing to speak with anyone about the disappearance. She eventually married again and went on to unparalleled literary success, including becoming a fellow of the Royal Society of Literature in 1950 and being appointed Commander of the Order of the British Empire (CBE) in the 1956. Several of her books were even made into films.

Despite all this, she maintained a muteness about her disappearance until her death in 1976. She refused to speak or even acknowledge the events of December 1926. Even her autobiography, published in 1977, makes no mention of these 11 days.

But that hasn’t stopped people from trying to unravel the mystery of the world’s most famous mystery writer. Some, including her biographer Morgan, believe she disappeared during a “fugue state,” a rare psychiatric phenomenon characterized by reversible amnesia for one’s identity, including the memories, personality, and other identifying characteristics, usually brought on severe stress or trauma. The impending divorce as well as recent loss of Christie’s mother during this time certainly lends credibility to this theory.

The author Jared Cade takes a more cynical approaching, believing that Christie planned the event to embarrass her husband but did not anticipate the resulting public melodrama. Another Christie biographer, Laura Thompson, theorized that Christie disappeared during a nervous breakdown, conscious of her actions but not in emotional control of herself.

Whatever the case, the mystery of Mrs. Christie–both on the page and off–is one that continues to fascinate readers over 100 years after her name first became synonymous with the genre.