The first recorded case occurred in 1817 with a reverend who, after days of suffering untold agony from toothache, experienced: "... all at once a sharp crack, like a pistol shot, bursting his tooth to fragments, [which] gave him instant relief. At this moment he turned to his wife, and said, 'My pain is all gone.' He went to bed, and slept soundly all that day and most of the succeeding night; after which he was rational and well."
This was the template for every case of exploding tooth syndrome (as we're now calling it). Victims would suffer from a tremendous toothache, followed by their mouth detonating from the inside out like baby aliens were inside. In 1830, a Mrs. Letita D reported an aching tooth "terminating by bursting with report," while a dentist in 1871 reported an occurrence of ETS so violent that the patient was knocked to the floor and deafened. Several similar cases later, however, and the condition vanished, never to be seen again.
As did the patients.
According to a group of researchers, it's likely that ETS was caused by a reaction between hydrogen gas and the metals used in old-timey fillings. In those days, fillings were often made from any combination of lead, tin, and silver, and it's possible that these could have created a low-voltage electrochemical cell (a battery, basically). The hydrogen, meanwhile, could have been created by any part of the tooth cavity that was accidentally left over from the often-poor dental surgery of the time. If the filling created an electrical charge in the presence of hydrogen, a miniature explosion could have occurred, with the tooth providing a handy bomb-like casing. People were essentially walking around with big fat mouthfuls of little Hindenburgs waiting to go off, which they did with terrifying regularity.