This is a mystery that has fascinated the scientific community for a very long time. On the night of May 5, 1100, the Moon gradually faded away to the point where it looked as if it had completely disappeared. In other words, this was an impressive and unexplained lunar eclipse.In-depth researchThe most useful testimony is that of a man in a manuscript called the Peterborough Chronicle, brought to light in various Anglo-Saxon monasteries in an attempt to tell the story of England between 1066 and the end of the 14th century. It describes that the Moon was 'so completely extinguished withal, that neither light, nor orb, nor anything at all of it was seen.' After a long investigation, the results of which were published in the Nature journal, researchers from the University of Geneva have unravelled this mystery. For a long time, scientists around the world assumed that this phenomenon was due to a huge volcanic eruption. The main suspect was Mount Helka in Iceland. Its eruption in 1104 could have formed a very thick cloud.The real reason? A volcano... in JapanSébastien Guillet, from the University of Geneva, has carried out a lot of research, in particular by studying ice cores in Greenland. He found traces of sulphate dating from 1107 to 1113, too late to be the remains of the Icelandic volcano's eruption.The volcanic eruption hypothesis seems to be a match, but it's not Mount Helka. Thanks to the testimonies of the time, one suspect stands out: Mount Asama, in Japan. Many people revealed that this Japanese volcano had experienced significant activity at the time. In particular, a high-ranking civil servant explained, according to statements reported by researchers, what happened:On August 29th, there was a fire at the top of the volcano, a thick layer of ash in the governor’s garden, everywhere the fields and the rice fields are rendered unfit for cultivation. We have never seen this in the country. It is a very strange and rare thing.By studying Japanese tree rings, Sebastian Guillet was able to confirm that there was a major volcanic eruption on Mt. Asama in 1108. This further validates the volcanic eruption as the reason why the moon disappeared in 1100 behind a thick cloud of sulphate.