“My dear Clara,” Major Archie Butt wrote in the summer of 1911, “It seems that the White House is haunted.” So began what would become the only written record of the mysterious Executive Mansion ghost known only as "The Thing."

That July, word of an apparition appearing to servants in William Howard Taft’s White House reached Butt, a military aide to the president who served as a kind of personal secretary and attaché. Reported encounters with a ghost had been scaring domestic staffers for months, as he recounted in a letter to his sister-in-law Clara. The spooky tale he told her remains an enduring question mark for White House scholars even today.

As the gossip of the time went, The Thing was felt more often than seen. Taft’s housekeeper—“a spooky little thing herself,” as Butt put it—reported that servants told stories of feeling The Thing appear as a slight pressure on the shoulder, as if a curious kid were leaning over to see what they were doing. Butt scolded the housekeeper, telling her, "ghosts have not the sense of touch, at least those self-respecting ghosts of which I have heard." But the servants maintained that it was, in fact, the spirit touching them.

“There’s a long tradition of White House ghosts,” Evan Phifer, a research historian at the White House Historical Society, tells Mental Floss. “Lincoln is a very popular one, Andrew Jackson—even a British soldier from the time of the War of 1812.” But The Thing is one of the more unusual White House spirits, because no one knows who he was. “It’s not a president or a first lady. It’s this unknown boy about 14 or 15 years old," Phifer says.

Several of the White House staff reported feeling this mysterious pressure on their shoulder, only to turn around to an empty room. Just one member of the household, though, said she actually saw the ghost. Marsh, First Lady Helen Taft’s personal maid, reported not just feeling the ghost leaning over her shoulder, but seeing the ethereal figure, whom she described as a young boy with light, unkempt hair and sad blue eyes. “Now who on Earth this can be,” Butt mused, “I cannot imagine.”

Taft responded to news of the spooky rumor with “towering rage,” Butt said, banning anyone in the house from speaking of the ghost under threat of firing. The president worried that the story would get out and the press would have a field day with the news. But his aide seemed to have a sense of humor about the whole situation. “I reminded him that the help was in such a state of mind that, if it was positively believed that the upper floor of the White House was haunted, the servants there could not be kept in their places by executive order,” Butt wrote.

Still, both of them found their curiosity piqued. Taft was “as anxious to hear about the thing as I had been,” according to Butt. And the aide, who was often on the receiving end of the housekeeping staff’s complaints, was afraid to let on just how intrigued he was by The Thing. “I don’t dare let any of them see how interested I am in it,” he told his sister-in-law. He hoped that the ghost would fade into the background as the year wore on and the staff got busier, relieving him of the duty of calming down superstitious domestic employees.

But even while publicly scoffing at the story, Butt was privately planning to research the mysterious boy’s possible origins. He asked several different servants to tell him their stories about The Thing, and told Clara that he was “going to delve into the history of the White House” to see if any boy matching The Thing’s description had lived—or died—there. But he never mentioned it in his letters to Clara again. It seems that he never did find out who the ghost might have been.

Modern White House historians are just as perplexed as he was. The only known youngster said to haunt the presidential residence is Willie Lincoln, who died during his father’s second year in office, possibly of typhoid fever. But Willie was 11 when he died, much younger than the description of The Thing. (Besides, Willie's ghost was already a known figure in Washington—the first reported sightings of his ghost in the White House date back to the 1870s.)

Whoever the paranormal figure might have represented, Taft was seemingly successful in squashing the rumors before they reached beyond the White House walls. “I didn’t really see the story in any papers of the time, so you could say that Archie Butt did a good job of keeping the story under wraps,” Phifer says. “This seems to be the only mention in the historical record of this ghost.”

Butt himself, though, was not long for this world. In April 1912, returning from Europe to the U.S. after a six-week leave of absence from the White House, he died in the sinking of the Titanic. And as for The Thing? Well, if Archie Butt ever did get to the bottom of the mystery, he took the story to his watery grave.