Great Hinamizawa Disaster
The Great Hinamizawa Disaster (雛見沢大災害, Hinamizawa Daisaigai) was a catastrophic gas leak incident in the village of Hinamizawa. The village's entire populace was exposed to a mixture of hydrogen sulfide and carbon dioxide and was completely wiped out overnight. The gas leak resulted in the permanent closing of visitation to the village.
The Great Hinamizawa Disaster does not happen on a specific day in each fragment, its occurrence ranges from June 21 to June 26.
The Great Hinamizawa Disaster first appears in Higurashi no Naku Koro ni in Tatarigoroshi-hen, however it can be assumed that the disaster occurs in the preceding chapters Onikakushi-hen and Watanagashi-hen shortly after each story concludes. However, the Great Hinamizawa Disaster never occurs in Meakashi-hen, the answer arc for Watangashi-hen. It can be inferred from this that the Disaster never occurs if Rika is killed by someone other than Miyo Takano.
The Official Story
The official version of the events, as per the reports of members of the Japan Special Defense Forces (JSDF) states that volcanic gas composed of hydrogen sulfide and carbon dioxide was released from Onigafuchi swamp. The gas spread through the village, killing everyone while they slept. The JSDF removed the bodies, quarantined the area for almost 20 years, and covered Onigafuchi Swamp with thick concrete. The event remains ruled as a natural disaster.
In the aftermath of the disaster a number of theories began popping up online. The most popular theory is one of a government conspiracy covering up an alien bio-terrorist attack. Multiple points are cited in support of this theory: First of all, experts say that filling the swamp with concrete was a pointless act, as it did no good against the gas. Additionally, hydrogen sulfide should have corroded metal and caused significant environmental damage in the town, but there was no evidence of this. The SDF soldiers who worked in the area after the disaster received frequent blood tests, which the conspiracy theorists think give evidence that they were testing some sort of bacteria on humans.
The primary support for the alien theory comes from File No.34 and the words of Rena Ryuuguu from the night prior to the incident, though this file only appears in Tsumihoroboshi-hen.