Washington D.C. can get sizzling hot in the summer — even when it isn’t an election year. That’s as true now as it was 100 years ago, before the advent of air conditioning. So when President Grover Cleveland announced he’d be out of town for a few weeks in the summer of 1893, it was no big surprise. The President made plans to sail on a friend’s yacht to his summer home on Cape Cod, and perhaps do a little fishing on the way. What he didn’t say was that while on board ship, he would undergo a secret operation to treat his oral cancer.

Why all the secrecy? For one thing, as author Matthew Algeo explains in his book The President Is a Sick Man, the U.S. economy was in a perilous state, and Cleveland worried that news about his health would upset Wall Street even further. What’s more, the stigma surrounding a cancer diagnosis was far worse than it is today, and there were few effective treatments. In fact, not even a decade before, oral cancer had claimed the life of president Ulysses S. Grant.

Several weeks before the trip, Cleveland (who made no secret of his fondness for alcohol and fine cigars) had noticed a swelling on the roof of his mouth. When he finally had it examined, the diagnosis was oral cancer. That’s when he secretly arranged the excursion, and recruited a team of six dentists and doctors to perform the operation onboard the yacht. All swore to remain silent… and not even the Vice President was let in on the plan.

The procedure was performed in the yacht’s salon, which had been converted into an operating room, on July 1, 1873. The medical team first anesthetized President Cleveland, and then removed a part of his upper jaw, along with five teeth. The 90-minute operation was successful, and left no noticeable scarring on his face — even sparing his distinctive moustache. In the next few weeks, as the President recovered, he was fitted with a rubber prosthesis that allowed him to eat and speak normally. After his treatment, Cleveland lived another 15 years… and in all that time, no one was the wiser.

Almost no one, that is. A few months afterward, a reporter named E.J. Edwards got wind of the clandestine operation, and published a story about it. But Cleveland, who had a reputation for honesty, denied it, and the story was discredited. The truth didn’t come out until 1917, when one of the doctors set the record straight in The Saturday Evening Post. That published account marked the end of one of the best-kept secrets of the U.S. Presidents — at least, so far as we know…