The Dyatlov Incident:
"The Dyatlov Pass incident refers to an event that resulted in the deaths of nine ski hikers in the northern Ural mountains. The incident happened on the night of February 2, 1959 on the east shoulder of the mountain Kholat Syakhl (Холат Сяхл) (a Mansi name, meaning Mountain of the Dead). The mountain pass where the incident occurred has been named Dyatlov Pass (Перевал Дятлова) after the group's leader, Igor Dyatlov (Игорь Дятлов).
The lack of eye witnesses to the circumstances of the tragedy, and subsequent journalistic investigations into the hikers' deaths, have inspired much speculation. Investigators determined that the hikers tore open their tent from within, departing barefoot in heavy snow. Though the corpses showed no signs of struggle, two victims had fractured skulls, two had broken ribs, and one was missing her tongue. According to sources, four of the victims' clothing contained high levels of radiation—though this was likely added at a later date, since no mention is made of it in contemporary documentation, and only appears in later documents. Soviet investigators determined only that "a compelling unknown force" had caused the deaths. Access to the area was barred for three years after the incident. The chronology of the incident remains unclear due to the lack of survivors." Read more.
Dancing Plague of 1518:
"The Dancing Plague (or Dance Epidemic) of 1518 was a case of dancing mania that occurred in Strasbourg, France (then part of the Holy Roman Empire) in July 1518. Numerous people took to dancing for days without rest, and over the period of about one month, most of the people died from heart attack, stroke, or exhaustion.
The outbreak began in July 1518, when a woman, Frau Troffea, began to dance fervently in a street in Strasbourg. This lasted somewhere between four to six days. Within a week, 34 others had joined, and within a month, there were around 400 dancers. Most of these people eventually died from heart attack, stroke, or exhaustion.
Historical documents, including "physician notes, cathedral sermons, local and regional chronicles, and even notes issued by the Strasbourg city council" are clear that the victims danced. It is not known why these people danced to their deaths, nor is it clear that they were dancing willfully." Read more.